We also need to invalidate the OpenGL/ES window when we resize the
window via a mouse drag operation, so that we don't get glitches in such
situations, because they are not covered in GdkWindow's
impl_class->move_resize().

Make sure that we only force the invalidation when necessary (as it is
expensive), and clean up the gdkevents-win32.c code so that we include
gdkglcontext-win32.h in the right place instead of using an extern, as
we need to invalidate the window accordingly.