The the user hasn't called SwapBuffers explicitly, we should not write to
the window surface.  This is especially a problem when the native window has
already been destroyed.

BUG=angleproject:2464

Change-Id: Ib4289de471ba2ad10e5cc21a8c2af946642f138c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1586355Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>