new SCREEN.mex (fixed offscreen bug)

dear steven

i think i found and fixed the bug you reported with the following
one-line program:
screen(-1,'OpenOffscreenWindow',0,[0 0 512 512]);
(A temporary rect variable wasn't initialized; the problem only occurred
with a screenNumber of -1.) Everything seems to work fine now.

Could you try it in your application and let me know?

SCREEN.mex is enclosed.

best

denis