Correctly adjust clip regions that lie offscreen

We were clamping the x/y location of the scissor to 0,0, but not adjusting
the width/height appropriately. This fix adjusts width/height and also clamps
them to 0 to correctly clip out offscreen operations.

Issue #7221524 Top left and top right portions of the screen blanks out after some time

Change-Id: I47f23336ea612409ed86652b9a68e272819ef00e