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>

include/SDL3/SDL_hints.h

@@ -1145,6 +1145,18 @@ extern "C" {
  */
 #define SDL_HINT_GPU_DRIVER "SDL_GPU_DRIVER"
 
+/**
+ * A variable that specifies the library name to use when loading the OpenXR loader.
+ *
+ * By default, SDL will try the system default name, but on some platforms like Windows,
+ * debug builds of the OpenXR loader have a different name, and are not always directly compatible with release applications.
+ * Setting this hint allows you to compensate for this difference in your app when applicable.
+ *
+ * This hint should be set before the OpenXR loader is loaded.
+ * For example, creating an OpenXR GPU device will load the OpenXR loader.
+ */
+#define SDL_HINT_OPENXR_LIBRARY "SDL_OPENXR_LIBRARY"
+
 /**
  * A variable to control whether SDL_hid_enumerate() enumerates all HID
  * devices or only controllers.