wayland: Set/unset the opaque regions on surfaces when transparency is toggled

Caches the SDL_HINT_VIDEO_EGL_ALLOW_TRANSPARENCY hint at init time and registers a callback, which is fired when the hint is changed during runtime and toggles the opaque region for existing surfaces.

src/video/wayland/SDL_waylandvideo.h

@@ -95,6 +95,7 @@ typedef struct {
     char *classname;
 
     int relative_mouse_mode;
+    SDL_bool egl_transparency_enabled;
 } SDL_VideoData;
 
 struct SDL_WaylandOutputData {