win32: Make flush/sync work like in X
win32: Make flush/sync work like in X

gdk_flush() should gdk_display_sync() on all open displays.

Both for display_flush and display_sync it seems useful to call
GdiFlush, but we don't have anything extra to do for display_sync,
as there is no inherent roundtrip on win32.

This should close bug #84314

Distrotech/gtk,johne53/MB3Gtk-2,Unity-Technologies/gtk,Unity-Technologies/gtk,johne53/MB3Gtk-2,Distrotech/gtk,Unity-Technologies/gtk,Unity-Technologies/gtk,Distrotech/gtk,johne53/MB3Gtk-2,Unity-Technologies/gtk,Distrotech/gtk,Distrotech/gtk,johne53/MB3Gtk-2,johne53/MB3Gtk-2