Split the keyboard onto its own translucent OpenXR quad

Expand the on-screen keyboard to a full US layout (letters, digits,
punctuation, Shift layer, Tab, arrows, F1-F12, one-shot Ctrl/Alt
modifiers), make it a persistent, movable, closeable panel via a
title-bar drag handle, and lower its opacity.

Previously the keyboard was just a second panel swapped into the same
quad as the menu launcher, which made the launcher translucent too and
made the keyboard and menu mutually exclusive. Extract the shared
swapchain/JNI/touch-dispatch plumbing into two generic, reusable
modules - xr_swapchain.c (swapchain + per-image FBO setup) and
xr_overlay.c (an off-screen Android View rendered into its own OpenXR
quad, hit-tested via a laser pointer) - and make the menu launcher and
keyboard two independent XrOverlay instances instead of one. The
controller's menu button now only opens/closes the launcher; the
launcher's "Keyboard" button only opens the keyboard - so the keyboard
can stay up while picking something from the menu, and only the
keyboard's own Close button hides it.

xr_menu.c/.h are gone, fully absorbed into xr_overlay.c. On the Java
side, OverlayPanel.java carries the shared off-screen-render/touch/
cursor plumbing that MenuOverlay and the new KeyboardOverlay both
subclass.

android/app/src/main/cpp/xr_session.h

@@ -9,6 +9,8 @@
 
 #include <stdbool.h>
 
+#include "xr_overlay.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -17,13 +19,25 @@ extern "C" {
 // current the GL context SDL/gl4es already use - creates the OpenXR
 // instance/session sharing that same EGL display/context, plus the game
 // quad's own swapchain, sized to the game's logical resolution
-// (game_width/height, i.e. grd_cap->w/h - see Shock.c's InitSDL()). The
-// menu quad's own, independently-sized swapchain (see xr_menu.h) is created
-// alongside it. 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.
+// (game_width/height, i.e. grd_cap->w/h - see Shock.c's InitSDL()). Also
+// creates the menu launcher and keyboard overlay quads (see xr_overlay.h) -
+// two independent XrOverlay instances, each with their own swapchain,
+// visibility, and position, retrievable via xr_session_get_menu_overlay()/
+// xr_session_get_keyboard_overlay() below. 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);
 
+// The menu launcher (small "Keyboard" button panel, toggled by the
+// controller's menu button) and keyboard (the on-screen key grid, shown/
+// hidden independently via MenuOverlay's "Keyboard" button and
+// KeyboardOverlay's own Close button) overlay quads - used by xr_input.c
+// for ray/quad hit-testing and touch/cursor dispatch, and by
+// questshock_native.c's JNI glue (nativeShowKeyboard()/
+// nativeRequestClose()). NULL if xr_init() failed before creating them.
+XrOverlay *xr_session_get_menu_overlay(void);
+XrOverlay *xr_session_get_keyboard_overlay(void);
+
 // 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).
@@ -44,10 +58,12 @@ bool xr_is_session_running(void);
 bool xr_frame_begin(void);
 
 // Releases the game quad's swapchain image (if one was acquired this
-// frame), acquires/renders/releases the menu quad's own swapchain image if
-// it's currently visible (see xr_menu.h), submits whichever of the two
-// quads actually rendered this frame as composition layers positioned in
-// front of the local reference space's origin, then ends the XR frame.
+// frame), then does the same acquire/render/release cycle for the menu
+// launcher and keyboard overlay quads via xr_overlay_render_and_build_layer()
+// for whichever of them is currently visible, and submits whichever of the
+// three quads actually rendered this frame as composition layers
+// positioned in front of the local reference space's origin, then ends the
+// XR frame.
 void xr_frame_end(void);
 
 void xr_shutdown(void);