st/mesa: fix int->uint conversion for negative scissor bound values

st/mesa: fix int->uint conversion for negative scissor bound values

Based on a patch by Xavier Chantry <chantry.xavier@gmail.com>:

If x+width or y+height is negative, then maxx or maxy will get a bogus value
when converting that to unsigned. Fix this by setting 0 as minimal value.

This was also triggered by teeworlds, but only with some combination of
resolution and map section. For example upper part of dm2 at 1280x1024.