Fix #109739: Resolve greyed out UI on MacOS

UI would sometimes disappear with the Metal backend.
This occured in situations where there was only a single
GPUContext within a single GHOST window, meaning
the activation routine for the context was never run.

Context activation is required to ensure the backbuffer
was up to date upon resize. Otherwise, re-sizing the
window would cause the screen to turn grey as the
new drawables would not be rendered to.

Authored by Apple: Michael Parkin-White

Co-authored-by: Michael Parkin-White <mparkinwhite@apple.com>
Pull Request: https://projects.blender.org/blender/blender/pulls/119299

source/blender/windowmanager/intern/wm_draw.cc

@@ -1544,6 +1544,13 @@ void wm_draw_update(bContext *C)
 
   BKE_image_free_unused_gpu_textures();
 
+#ifdef WITH_METAL_BACKEND
+  /* Reset drawable to ensure GPU context activation happens at least once per frame if only a
+   * single context exists. This is required to ensure the default framebuffer is updated
+   * to be the latest backbuffer. */
+  wm_window_clear_drawable(wm);
+#endif
+
   LISTBASE_FOREACH (wmWindow *, win, &wm->windows) {
 #ifdef WIN32
     GHOST_TWindowState state = GHOST_GetWindowState(