Scissors provide a critical hint to tiled renderers as to what tiles
need to be load/stored because they could be modified by the
rendering.

The bounds calculation here is limited to when we have a small number
of rects (large enough to cover rounded window corners, but probably
not xeyes) to avoid overhead on desktop GL.

No performance difference on i965 with x11perf -rect1 -repeat 1 -reps
10000 (n=50)

v2: Clamp rectangle bounds addition.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Keith Packard <keithp@keithp.com>