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