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/cpp/xr_session.h

@@ -0,0 +1,56 @@
+// Minimal OpenXR bring-up for questshock's immersive Quest build. Keeps the
+// game's own rendering untouched (see android/engine-patches/
+// 12-android-openxr-present.patch) - this module only owns the OpenXR
+// instance/session/swapchain and the final "submit a quad layer instead of
+// presenting to a window" step. No stereo rendering, no controller input
+// yet (see the plan's steps B/C/D for those) - the game composite is shown
+// as a single flat quad floating in front of the viewer.
+#ifndef QUESTSHOCK_XR_SESSION_H
+#define QUESTSHOCK_XR_SESSION_H
+
+#include <stdbool.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// Call once, right after init_opengl() (OpenGL.cc) has created and made
+// current the GL context SDL/gl4es already use - creates the OpenXR
+// instance/session sharing that same EGL display/context, plus one
+// swapchain sized to the game's logical resolution (game_width/height, i.e.
+// grd_cap->w/h - see Shock.c's InitSDL()). Returns false if OpenXR bring-up
+// failed (e.g. no runtime installed) - callers should fall back to the
+// existing window-present path in that case.
+bool xr_init(int game_width, int game_height);
+
+// Pumps XR session-state events. Call once per frame, before
+// xr_frame_begin(). Must still be called even when xr_init() returned
+// false (no-op in that case).
+void xr_poll_events(void);
+
+// True once the session has reached a state where frames are expected
+// (XR_SESSION_STATE_SYNCHRONIZED or later) - i.e. it's safe/required to
+// start calling xr_frame_begin()/xr_frame_end() each frame.
+bool xr_is_session_running(void);
+
+// Begins the XR frame (xrWaitFrame/xrBeginFrame) and, if the runtime wants
+// this frame rendered, acquires the next swapchain image and binds its
+// framebuffer as the current render target - ready for the caller to draw
+// into exactly as it would have drawn to the default framebuffer. Returns
+// true if the caller should draw this frame; xr_frame_end() must be called
+// unconditionally afterward either way (a begun XR frame must always be
+// ended, rendered or not).
+bool xr_frame_begin(void);
+
+// Releases the swapchain image (if one was acquired this frame) and
+// submits it as a single XrCompositionLayerQuad positioned in front of the
+// local reference space's origin, then ends the XR frame.
+void xr_frame_end(void);
+
+void xr_shutdown(void);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif