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.

android/app/src/main/AndroidManifest.xml

@@ -12,6 +12,11 @@
     <!-- External mouse input events -->
     <uses-feature android:name="android.hardware.type.pc" android:required="false" />
 
+    <!-- Immersive OpenXR app now (see android/app/src/main/cpp/xr_session.c),
+         not a 2D Home panel - vr.headtracking is what actually declares that
+         to Horizon OS. -->
+    <uses-feature android:name="android.hardware.vr.headtracking" android:version="1" android:required="true" />
+
     <!-- Plain, unrestricted /sdcard access for the res/data,res/sound the
          user drops in and the shaders/soundfont this app extracts there on
          first run - see res/GET_ASSETS_QUEST.txt and QuestShockActivity.
@@ -28,6 +33,11 @@
         android:theme="@android:style/Theme.NoTitleBar.Fullscreen"
         android:hardwareAccelerated="true" >
 
+        <!-- Tells Horizon OS this activity wants the compositor for itself
+             (immersive VR), not the Home 2D-panel path - required for an
+             OpenXR session to actually start. -->
+        <meta-data android:name="com.oculus.vr.focusaware" android:value="true" />
+
         <activity android:name="de.ladkau.questshock.QuestShockActivity"
             android:label="@string/app_name"
             android:alwaysRetainTaskState="true"
@@ -40,23 +50,19 @@
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />
                 <category android:name="android.intent.category.LAUNCHER" />
+                <!-- The vendor-neutral, Khronos-defined way to declare an
+                     immersive OpenXR entry point (vs. e.g. Meta's own
+                     com.oculus.intent.category.VR) - see this project's
+                     "favor OpenXR" rule in the README. If Horizon OS still
+                     launches this as a flat panel on-device, the next thing
+                     to try is adding a com.oculus.supportedDevices meta-data
+                     listing the target Quest models - not confirmed
+                     necessary from static analysis alone. -->
+                <category android:name="org.khronos.openxr.intent.category.IMMERSIVE_HMD" />
             </intent-filter>
             <intent-filter>
                 <action android:name="android.hardware.usb.action.USB_DEVICE_ATTACHED" />
             </intent-filter>
-            <!-- Without this, Quest's Home shell picks its own default 2D
-                 panel shape for this activity - a tall portrait rectangle,
-                 which crops a 4:3 landscape game like System Shock's classic
-                 640x480 down to a sliver. defaultWidth/defaultHeight (the
-                 standard Android multi-window sizing hint, which Quest's
-                 panel system honors for 2D apps) requests a properly
-                 landscape, 4:3-ish panel instead; minWidth/minHeight keep it
-                 from being resized below the game's native resolution. -->
-            <layout android:defaultWidth="1280dp"
-                android:defaultHeight="960dp"
-                android:minWidth="640dp"
-                android:minHeight="480dp"
-                android:gravity="center" />
         </activity>
     </application>