a3xx: use window scissor to simulate viewport xy clip

a3xx: use window scissor to simulate viewport xy clip

Unfortunately a3xx does not have a separate disable for depth clipping,
so when depth clamp is enabled, we disable the whole 3d clipper logic.
This in turn also gets rid of the xy clip that it would normally do.
When we detect this would happen, instead we integrate the viewport into
the window scissor. This may have slightly different behavior around
wide points, but it's unlikely that anything depends on this.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97231
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: mesa-stable@lists.freedesktop.org