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/questshock_native.c

@@ -9,6 +9,9 @@
 
 #include <SDL.h>
 
+#include "xr_overlay.h"
+#include "xr_session.h"
+
 JNIEXPORT void JNICALL
 Java_de_ladkau_questshock_QuestShockActivity_nativeChdir(JNIEnv *env, jclass clazz, jstring path) {
     const char *cpath = (*env)->GetStringUTFChars(env, path, NULL);
@@ -16,17 +19,18 @@ Java_de_ladkau_questshock_QuestShockActivity_nativeChdir(JNIEnv *env, jclass cla
     (*env)->ReleaseStringUTFChars(env, path, cpath);
 }
 
-// MenuOverlay's hand-built on-screen keyboard (see
-// MenuOverlay.commitPrintableChar()) has no real Android IME session behind
-// it, so printable characters can't reach the game via Android's usual IME
-// path - instead this builds and pushes an SDL_TEXTINPUT event directly,
-// the same event type/shape SDL's own Android backend pushes for typed
-// text, using only the public SDL_Event/SDL_PushEvent API. The engine's
-// pump_events() (sdl_events.c) only picks up printable characters from
-// this event, not from SDL_KEYDOWN (see handleKeyPress() in MenuOverlay.java
-// for why).
+// KeyboardOverlay's hand-built on-screen keyboard (see
+// KeyboardOverlay.commitPrintableChar()) has no real Android IME session
+// behind it, so printable characters can't reach the game via Android's
+// usual IME path - instead this builds and pushes an SDL_TEXTINPUT event
+// directly, the same event type/shape SDL's own Android backend pushes for
+// typed text, using only the public SDL_Event/SDL_PushEvent API. The
+// engine's pump_events() (sdl_events.c) only picks up printable characters
+// from this event, not from SDL_KEYDOWN (see handleKeyPress() in
+// KeyboardOverlay.java for why).
 JNIEXPORT void JNICALL
-Java_de_ladkau_questshock_MenuOverlay_nativeSendPrintableChar(JNIEnv *env, jclass clazz, jchar c) {
+Java_de_ladkau_questshock_KeyboardOverlay_nativeSendPrintableChar(JNIEnv *env, jclass clazz,
+                                                                   jchar c) {
     SDL_Event event;
     SDL_zero(event);
     event.type = SDL_TEXTINPUT;
@@ -36,3 +40,20 @@ Java_de_ladkau_questshock_MenuOverlay_nativeSendPrintableChar(JNIEnv *env, jclas
     event.text.text[1] = '\0';
     SDL_PushEvent(&event);
 }
+
+// KeyboardOverlay's title-bar Close button - the keyboard is a fully
+// independent overlay quad from the menu launcher (see xr_overlay.h), so
+// this is the only way to hide it once it's open; the controller's menu
+// button only ever affects the menu launcher.
+JNIEXPORT void JNICALL
+Java_de_ladkau_questshock_KeyboardOverlay_nativeRequestClose(JNIEnv *env, jclass clazz) {
+    xr_overlay_set_visible(xr_session_get_keyboard_overlay(), false);
+}
+
+// MenuOverlay's "Keyboard" button - only opens the keyboard overlay, never
+// touches the menu launcher's own visibility, so both can be shown
+// together.
+JNIEXPORT void JNICALL
+Java_de_ladkau_questshock_MenuOverlay_nativeShowKeyboard(JNIEnv *env, jclass clazz) {
+    xr_overlay_set_visible(xr_session_get_keyboard_overlay(), true);
+}