uxa: Reuse in-flight bo

When we need to allocate a new bo for use as a gpu target, first check
if we can reuse a pixmap that has already been relocated into the
aperture as a temporary target, for instance a glyph mask or a clip mask.

Before:
backend                      test   min(s) median(s) stddev.
xlib         firefox-planet-gnome   50.568   50.873   0.30%
 xcb         firefox-planet-gnome   49.686   53.003   3.92%
xlib                    evolution   40.115   40.131   0.86%
 xcb                    evolution   28.241   28.285   0.18%

After:
backend                      test   min(s) median(s) stddev.
xlib         firefox-planet-gnome   47.759   48.233   0.80%
 xcb         firefox-planet-gnome   48.611   48.657   0.87%
xlib                    evolution   38.954   38.991   0.05%
 xcb                    evolution   26.561   26.654   0.19%

And even more dramatic improvements when using a font size larger than
the maximum size of the glyph cache:
 xcb firefox-36-20090611:  1.79x speedup
xlib firefox-36-20090611:  1.74x speedup
 xcb firefox-36-20090609:  1.62x speedup
xlib firefox-36-20090609:  1.59x speedup

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

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