Fixes:

  Bug 28446 - Garbled Font with Mathematica 7
  https://bugs.freedesktop.org/show_bug.cgi?id=28446



Rewriting the glyphs to render to the destination directly and removing
the more expensive multiple invocations of CompositePicture per picture
was a great performance boost -- except that it needs special handling
in the backend in order to not fallback. Having done so for i915, I
neglected to ensure the sanity checking in i965_prepare_composite() was
sufficient. As it turns out, it was not and so we misrendered CA-glyphs
when rendering directly to the destination. This causes us to fallback
properly, but is a performance regression as we no longer try the 2-pass
magic helper before resorting to s/w. At the moment, I'd rather live
with the temporary regression and fix i965 to do the same magic as i915,
as it critical to fixing the severe performance issues currently
crippling i965, as I believe that this regression only affects the
minority of applications (incorrect, as it turns out, as the glyphs are
overlapping) rendering directly to the destination.

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