[PATCH, pixman] Use the destination buffer directly in more cases

[Soeren Sandmann]

When the destination buffer is either a8r8g8b8 or x8r8g8b8, we can use
it directly instead of fetching into a temporary buffer. When the
format is x8r8g8b8, we require the operator to not make use of
destination alpha, but when it is a8r8g8b8, there are no
restrictions.

This is approximately a 5% speedup on the poppler cairo benchmark:
    
[ # ]  backend                         test   min(s) median(s)    stddev. count
    
Before:
[  0]    image                      poppler    6.661    6.709     0.59%    6/6
    
 After:
[  0]    image                      poppler    6.307    6.320     0.12%    5/6

The patch is here:

   http://cgit.freedesktop.org/~sandmann/pixman/commit/?h=direct-destination

Thanks,
Soren