[cairo] glitz surface fixes

[Vladimir Vukicevic]

The attached patch fixes the cairo glitz surface; the culprit seemed
to be that glitz (now?) applies the clip region when glitz_get_pixels
is called; this is not what acquire_*_image expects, and so bad
results happen in both writing a png out from a glitz surface, and any
fallback paths that have to be taken that require pixman compositing.

There's also a patch to the image surface, which attempts to find a
standard pixman format given a set of masks.  This is necessary so
that the generic cairo_surface_write_to_png() will work with glitz
surfaces; glitz surfaces use _cairo_image_surface_create_with_masks()
to create an image surface, and the png code would bail because it
didn't recognize the cairo_format_t.

All tests pass except for two -- there's a problem with unbounded
operators and glyph rendering, and self-copy also fails.  (The
unantialiased rendering tests also fail; I think the fix there is to
just use a fallback path if non-AA rendering is desired.)  A number of
tests pass but are marked as fails; there are a lot of different
pixels (especially in rendered gradients), but the results are
actually correct.  Not sure what to do about this in the test
framework; there may be a need for per-backend reference images.

David, Carl, is it ok to commit this if it looks correct?

    - Vlad
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