gdkwindow-win32.c: Fix GLES glitches during un-fullscreen
During the attempt to work out a workaround fix for issue #1702, glitches showed up when we attempt to un-fullscreen a window that is using GLES/libANGLE. This MR attempts to fix such gliches. From the commit comments:
We also need to force redraw of the whole window when we are using EGL/ANGLE during un-fullscreen, so that we do not get glitches in the resulting window.
With blessings, thank you!