Use the destination buffer directly in more cases instead of fetching.

Use the destination buffer directly in more cases instead of fetching.

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