GPU: OpenXR integration (#14837)

Based on Beyley's initial draft in #11601.

Co-authored-by: Beyley Cardellio <ep1cm1n10n123@gmail.com>
Co-authored-by: Ethan Lee <flibitijibibo@gmail.com>

src/dynapi/SDL_dynapi.sym

@@ -1272,6 +1272,13 @@ SDL3_0.0.0 {
     SDL_LoadSurface;
     SDL_SetWindowFillDocument;
     SDL_TryLockJoysticks;
+    SDL_CreateGPUXRSession;
+    SDL_GetGPUXRSwapchainFormats;
+    SDL_CreateGPUXRSwapchain;
+    SDL_DestroyGPUXRSwapchain;
+    SDL_OpenXR_LoadLibrary;
+    SDL_OpenXR_UnloadLibrary;
+    SDL_OpenXR_GetXrGetInstanceProcAddr;
     # extra symbols go here (don't modify this line)
   local: *;
 };