st/mesa: fix unsigned/signed breakage in scissor

st/mesa: fix unsigned/signed breakage in scissor

commit 53174afeeb introduced a portability change that converted GLint x,y
to GLuint. That breaks when x and y are negative, which seems to be allowed,
and which at least one game uses : teeworlds.

Rather than simply reverting the change, it seems possible to convert the
16bit unsigned to GLint so that comparisons are made between signed integers
instead.  This hopefully does not break anything while keeping MSVC happy.

Signed-off-by: Xavier Chantry <chantry.xavier@gmail.com>
Signed-off-by: Brian Paul <brianp@vmware.com>