Add a noop composite function for the DST operator

Add a noop composite function for the DST operator

The DST operator doesn't actually do anything, so add a noop "fast
path" for it, instead of checking in pixman_image_composite32().

The performance tradeoff here is that we get rid of a test for DST in
the common case where the operator is not DST, in return for an extra
walk over the clip rectangles in the uncommon case where the operator
actually is DST.