Fix T97600 Regression: rendering in new window displays flickers

This is because some drivers / GPU actually still do double buffer swapping
but others don't. Adding this do ensure the background color of the first
redraw.

Note that this fix was not tested on the problematic hardware and might not
solve the issue.

source/blender/windowmanager/intern/wm_window.c

@@ -578,6 +578,9 @@ static void wm_window_ghostwindow_add(wmWindowManager *wm,
 
     wm_window_swap_buffers(win);
 
+    /* Clear double buffer to avoids flickering of new windows on certain drivers. (See T97600) */
+    GPU_clear_color(0.55f, 0.55f, 0.55f, 1.0f);
+
     // GHOST_SetWindowState(ghostwin, GHOST_kWindowStateModified);
   }
   else {