mesa/st: Passthrough scissor when clearing by quad

mesa/st: Passthrough scissor when clearing by quad

The scissor state -is- setup, but the scissor test is not enabled. This
can prevent certain optimizations from occurring on tilers where
unaffected tiles are thrown out entirely.

v2: Only enable scissor test if the scissor test is actually set by the
app, to avoid regressing quad-based clears used for other reasons (like
a color mask).

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>