Add controller input and a laser-pointer-driven menu quad

- xr_input.c/h: an OpenXR action set (aim pose, trigger/select click, a
  menu-toggle button) plus ray/quad hit-testing, and a visible
  laser-pointer line drawn from the controller toward the hit point.
- xr_menu.c/h + MenuOverlay.java: a second composition-layer quad,
  toggled by a controller button, showing an off-screen, never-attached
  Android View tree (one "Keyboard" button so far) driven by synthetic
  touch events computed from the laser ray's hit UV.
- Migrate gl4es from a prebuilt binary baked into the Docker build-image
  to a vendored source snapshot (android/gl4es-src/) compiled from
  source at APK build time via CMake add_subdirectory(), patched through
  a new android/gl4es-patches/ (mirrors the existing engine-patches/
  pattern). This was needed to track down and fix a bug hit while
  building the menu: gl4es's fixed-pipeline emulation (fpe.c) was
  unconditionally substituting its own shader onto the menu's draw call
  (fpe_ReleventState() always sets alphafunc to a nonzero sentinel, so
  its fpe_IsEmpty() check could never see the state as empty), making
  the menu quad show the game's own rendering instead of its own
  content. Fixed by routing the menu's blit through a real
  glBlitFramebuffer() call instead of a shader-based draw, which gl4es's
  fpe.c has nothing to intercept - documented in README.md's new
  "Debugging notes" section.
- Renumber android/engine-patches/ to close the gap left by removing an
  unrelated diagnostic-only patch, and strip investigation-journal
  comments and dead diagnostic code (temporary tracing, env-var probes)
  left over from finding the bug above.
- Restructure README.md into numbered sections, document every
  engine/gl4es patch, add Android Studio dev/testing-cycle instructions,
  and generalize Quest-specific wording to any OpenXR headset. Update
  NOTICE.txt to match (gl4es is now vendored/patched source, not a
  prebuilt binary; add the OpenXR-SDK loader).

build-image/Dockerfile

@@ -56,7 +56,21 @@ ARG ANDROID_16KB_LDFLAGS="-Wl,-z,max-page-size=16384"
 # GL4ES (https://github.com/ptitSeb/gl4es) - translates the engine's
 # desktop-style immediate-mode OpenGL calls into real GLES/EGL calls, so
 # engine/src/MacSrc/OpenGL.cc needs no immediate-mode rewrite on Android.
-ARG ANDROID_GL4ES_VERSION=1.1.6
+# Vendored directly into android/gl4es-src/ (currently tag v1.1.6, gl4es's
+# latest release) rather than fetched at build time, same rationale as
+# engine/: lets android/gl4es-patches/ instrument it directly (this is
+# Android/GLES-only, with no desktop build depending on it, so unlike
+# engine/ there's no shared tree to protect - see that patch's own comment
+# for why this was needed, tracking down the OpenXR menu-quad texture bug).
+# Re-vendoring a newer tag later: re-clone into android/gl4es-src/, then
+# reapply/update the patches in android/gl4es-patches/.
+# Unlike SDL2/SDL2_mixer/fluidsynth-lite/OpenXR below, gl4es is NOT
+# prebuilt in this image - it's compiled from a scratch, patched copy as a
+# CMake subdirectory of the engine's own native build (see
+# build-image/prepare-android-project.sh and
+# android/engine-patches/02-android-opengl-es.patch), so gl4es-patches/
+# changes only need a normal Android Studio/gradlew rebuild, not a
+# build-image rebuild.
 # Khronos' own OpenXR loader (https://github.com/KhronosGroup/OpenXR-SDK) -
 # the vendor-neutral loader, not Meta's redistributed copy, per this
 # project's "favor OpenXR" rule (see README's Design principles). Only the
@@ -92,17 +106,12 @@ ENV DEBIAN_FRONTEND=noninteractive
 # build cache for this layer - and vice versa, adding a package here only
 # ever costs a rebuild of the (slow) layers below it, not a re-download
 # of packages that didn't change.
-# patchelf: fixes up GL4ES's built-in ELF SONAME below (see the gl4es
-# build step) - the Android APK packager (AGP) refuses to bundle a
-# jniLibs file whose name doesn't literally end in ".so", but the
-# dynamic linker resolves NEEDED entries by embedded SONAME, not
-# filename - patchelf lets both agree on "libGL.so".
 RUN apt-get update && apt-get install -y --no-install-recommends \
         build-essential cmake make git curl ca-certificates pkg-config gosu \
         libgl1-mesa-dev libglx-dev libxext-dev libx11-dev libxrandr-dev \
         libxi-dev libxfixes-dev libxss-dev libxinerama-dev libxcursor-dev \
         libogg-dev libvorbis-dev libasound2-dev openssh-client \
-        openjdk-17-jdk-headless unzip patchelf \
+        openjdk-17-jdk-headless unzip \
     && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /opt/prebuilt
@@ -252,34 +261,6 @@ RUN git clone https://github.com/EtherTyper/fluidsynth-lite.git fluidsynth-lite-
         /opt/prebuilt/android/fluidsynth-lite/include/fluidsynth/version.h \
     && rm -rf fluidsynth-lite-android build-fluidsynth-android
 
-# gl4es's own CMakeLists.txt writes its output straight to
-# ${CMAKE_SOURCE_DIR}/lib (the source tree, not the build dir) and gives
-# the GL target a ".so.1" suffix, both in filename and in its embedded
-# ELF SONAME - stage explicitly rather than `cmake --install` (which it
-# doesn't support for this target anyway). Renaming the *file* to
-# "libGL.so" isn't enough on its own: the dynamic linker resolves
-# libmain.so's NEEDED entry by the *embedded* SONAME, not by whatever
-# filename it was linked against, so a bare rename would link fine but
-# fail to dlopen on-device. And keeping the real "libGL.so.1" filename
-# isn't an option either - AGP's jniLibs packaging silently drops any
-# native library whose filename doesn't literally end in ".so". So
-# patchelf the embedded SONAME to "libGL.so" too, making the rename
-# consistent both at link time and at runtime.
-RUN git clone --branch "v${ANDROID_GL4ES_VERSION}" --depth 1 \
-        https://github.com/ptitSeb/gl4es.git gl4es-android \
-    && rm -rf gl4es-android/.git \
-    && cmake -S gl4es-android -B build-gl4es-android \
-        -DCMAKE_TOOLCHAIN_FILE="${ANDROID_NDK_TOOLCHAIN}" \
-        -DANDROID_ABI="${ANDROID_ABI}" -DANDROID_PLATFORM="android-${ANDROID_PLATFORM_VERSION}" \
-        -DANDROID=ON -DUSE_ANDROID_LOG=ON -DSTATICLIB=OFF \
-        -DCMAKE_SHARED_LINKER_FLAGS="${ANDROID_16KB_LDFLAGS}" \
-    && cmake --build build-gl4es-android -j"$(nproc)" \
-    && mkdir -p /opt/prebuilt/android/gl4es/lib /opt/prebuilt/android/gl4es/include \
-    && cp -a gl4es-android/lib/libGL.so.1 /opt/prebuilt/android/gl4es/lib/libGL.so \
-    && patchelf --set-soname libGL.so /opt/prebuilt/android/gl4es/lib/libGL.so \
-    && cp -a gl4es-android/include/. /opt/prebuilt/android/gl4es/include/ \
-    && rm -rf gl4es-android build-gl4es-android
-
 # OpenXR loader, for the immersive VR mode (android/app/src/main/cpp/
 # xr_session.c and friends) - see android/engine-patches/
 # 11-android-openxr-cmake.patch for how the engine links it.