Start with a smaller default window size; avoid race conditions on resize.

Start with a smaller default window size; avoid race conditions on resize.

Because most mesa demos use a 250x250 window, and the window only grows
to accomodate the viewport (never shrinks) so we end up retracing with
a larger framebuffer.

But for this to work all the time, especially with DRI drivers which do
not hook into glViewport, it was necessary to do more event processing
to ensure that by the time the resize is done, the new window size is
visible to GL.