st/xa: scissor to help tilers

st/xa: scissor to help tilers

Keep track of the maximal bounds of all the operations and set scissor
accordingly.  For tiling GPU's this can be a big win by reducing the
memory bandwidth spent moving pixels from system memory to tile buffer
and back.

You could imagine being more sophisticated and splitting up disjoint
operations.  But this simplistic approach is good enough for the common
cases.

Signed-off-by: Rob Clark <robclark@freedesktop.org>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>