Partial black screen if update callback not defined

[dogwynn]

If the window dimensions extend beyond the end of the screen on first rendering (e.g. issue #247), and if there is not an update callback defined, then the the portion of the window outside of screen is not rendered. Furthermore, if the window is brought fully into view, the unrendered portion remains black.

pygame 2.0.1 (SDL 2.0.14, Python 3.9.1)
pgzero 1.2.1
Windows 10

[lordmauve]

I believe this is fixed in 126f7cf by handling the VIDEOEXPOSE event.