Change so that modifer keypresses will only register when the window has
focus in Windows. This corrects the issue with SLEEP, KEYHIT, and
INP(&H60) registering SHIFT, CTRL, and ALT keypresses even when the
program is not in focus or is running as a background app.
Also changed _SCREENEXISTS (and a few other window user commands) so
that they require windows to have actually registered a windows handle
for the function to return -1, and not just for glut to have finished
initializing the screen itself.