Add OpenXR bring-up: render the game as a floating quad in an immersive Quest session

- New android/app/src/main/cpp/xr_session.c (+.h): owns the OpenXR
  instance/session/local-space/swapchain and the per-frame
  xrWaitFrame/xrBeginFrame/xrEndFrame loop, submitting the game's
  existing flat render as a single head-tracked XrCompositionLayerQuad.
  No stereo rendering or controller input yet - that's steps B/C/D of
  the plan.
- 11-android-openxr-cmake.patch: links the OpenXR loader as a build
  dependency, staged into jniLibs the same way as gl4es/SDL2.
  build-image/Dockerfile and prepare-android-project.sh build and stage
  it.
- 12-android-openxr-present.patch: redirects OpenGL.cc's final present
  step (opengl_swap_and_restore / SDLDraw's software path) into the XR
  swapchain FBO instead of the window, on Android only. Two real gl4es
  bugs had to be worked around to get pixels on screen at all:
    - gl4es's own glBindFramebuffer errors on an FBO id it didn't create
      itself, even though the real driver-level bind succeeds - fixed by
      creating the swapchain FBO container via gl4es's own
      glGenFramebuffers/glBindFramebuffer, and only using the real
      (dlsym'd) driver call for attaching OpenXR's foreign swapchain
      texture, which gl4es's own attach can't handle.
    - gl4es's glBegin/glVertexAttrib/glVertex3f immediate-mode emulation
      only captures a fresh per-vertex value for attribute 0 (position,
      driven directly by glVertex3f) - custom attributes like this
      shader's texcoords/light are GLES2's *constant*-attribute API and
      applied once for the whole draw, not per vertex, silently
      collapsing the UI-overlay quad to a single sampled texel. Fixed by
      switching that one draw call to real vertex arrays
      (glVertexAttribPointer/glDrawArrays).
  Also flips the V texcoord to match SDL's top-down row order against
  GL's texture convention, and reuses a persistent texture object
  instead of a fresh gen/upload/delete every frame.
- AndroidManifest.xml: declares the immersive-HMD intent category and
  focus-aware metadata, drops the 2D-panel layout hint.
- README: documents the new OpenXR immersive mode.

build-image/Dockerfile

@@ -57,6 +57,14 @@ ARG ANDROID_16KB_LDFLAGS="-Wl,-z,max-page-size=16384"
 # 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
+# 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
+# loader is built (BUILD_LOADER=ON, everything else off) - the actual XR
+# runtime implementation is resolved on-device by Horizon OS at load time,
+# nothing else needs bundling. Verify this is still the latest release tag
+# at https://github.com/KhronosGroup/OpenXR-SDK/releases when bumping.
+ARG ANDROID_OPENXR_VERSION=1.1.42
 
 ENV DEBIAN_FRONTEND=noninteractive
 
@@ -272,6 +280,26 @@ RUN git clone --branch "v${ANDROID_GL4ES_VERSION}" --depth 1 \
     && 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.
+RUN git clone --branch "release-${ANDROID_OPENXR_VERSION}" --depth 1 \
+        https://github.com/KhronosGroup/OpenXR-SDK.git openxr-sdk-android \
+    && rm -rf openxr-sdk-android/.git \
+    && cmake -S openxr-sdk-android -B build-openxr-android \
+        -DCMAKE_TOOLCHAIN_FILE="${ANDROID_NDK_TOOLCHAIN}" \
+        -DANDROID_ABI="${ANDROID_ABI}" -DANDROID_PLATFORM="android-${ANDROID_PLATFORM_VERSION}" \
+        -DCMAKE_INSTALL_PREFIX=/opt/prebuilt/android/openxr \
+        -DBUILD_LOADER=ON -DBUILD_TESTS=OFF -DBUILD_API_LAYERS=OFF \
+        -DBUILD_CONFORMANCE_TESTS=OFF -DBUILD_WITH_SYSTEM_JSONCPP=OFF \
+        -DDYNAMIC_LOADER=ON \
+        -DCMAKE_SHARED_LINKER_FLAGS="${ANDROID_16KB_LDFLAGS}" \
+    && cmake --build build-openxr-android -j"$(nproc)" --target openxr_loader \
+    && cmake --install build-openxr-android \
+    && mkdir -p /opt/prebuilt/android/openxr/include \
+    && cp -a openxr-sdk-android/include/. /opt/prebuilt/android/openxr/include/ \
+    && rm -rf openxr-sdk-android build-openxr-android
+
 COPY build-image/docker-entrypoint.sh /usr/local/bin/docker-entrypoint.sh
 COPY build-image/build-engine.sh /usr/local/bin/build-engine.sh
 RUN chmod +x /usr/local/bin/docker-entrypoint.sh /usr/local/bin/build-engine.sh

build-image/prepare-android-project.sh

@@ -1,8 +1,8 @@
 #!/usr/bin/env bash
 # Prepares android/ to be built: a scratch, patched copy of engine/ (see
 # android/engine-patches/ - engine/ itself is never modified), the
-# Android SDL2/SDL2_mixer/fluidsynth-lite/gl4es prebuilts staged into
-# jniLibs, bundled assets, and android/engine.properties (engineDir/
+# Android SDL2/SDL2_mixer/fluidsynth-lite/gl4es/openxr prebuilts staged
+# into jniLibs, bundled assets, and android/engine.properties (engineDir/
 # prebuiltDir, read by android/app/build.gradle). Always run inside this
 # image (via ../run-image.sh, or directly if already inside it) - it
 # reads from /opt/prebuilt/android/*, which only exists there.
@@ -75,12 +75,12 @@ cp -a "$ANDROID_DIR/gles-shaders/." "$ASSETS_DIR/shaders/"
 cp "/opt/prebuilt/soundfont/default.sf2" "$ASSETS_DIR/res/soundfont.sf2"
 cp "$REPO_ROOT/res/assets/GET_ASSETS_QUEST.txt" "$ASSETS_DIR/GET_ASSETS_QUEST.txt"
 
-echo "== Staging prebuilt Android SDL2/SDL2_mixer/fluidsynth-lite/gl4es .so's into jniLibs =="
+echo "== Staging prebuilt Android SDL2/SDL2_mixer/fluidsynth-lite/gl4es/openxr .so's into jniLibs =="
 JNI_LIBS_DIR="$ANDROID_DIR/app/src/main/jniLibs/arm64-v8a"
 rm -rf "$ANDROID_DIR/app/src/main/jniLibs"
 mkdir -p "$JNI_LIBS_DIR"
 find /opt/prebuilt/android/sdl2/lib /opt/prebuilt/android/sdl2_mixer/lib /opt/prebuilt/android/fluidsynth-lite/lib \
-    /opt/prebuilt/android/gl4es/lib \
+    /opt/prebuilt/android/gl4es/lib /opt/prebuilt/android/openxr/lib \
     -name '*.so' -exec cp -a {} "$JNI_LIBS_DIR/" \;
 
 echo "== Done =="