xcb: Take advantage of clip-boxes

A demonstration of step 2, improves performance for selected benchmarks
on selected GPUs by up to 30%.

firefox-fishbowl on snb {i5-2520m): 42s -> 29s.
firefox-talos-gfx on snb: 7.6 -> 5.2s.
firefox-fishbowl on pnv (n450): 380 -> 360s.

Whist this looks like it is getting close to as good as we can achieve,
we are constrained by both our API and Xrender and fishbowl is about 50%
slower than peak performance (on snb).

And it fixes the older performance regression in firefox-planet-gnome.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

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