[Pixman] [RFC] A couple of minor tweaks for sse2 blitting

[Chris Wilson]

The generic implementation of the BLT routine is to deny the operation.
So we need to add support to the sse2 implementation in order to perform
8bpp. This looked relatively straightforward, so I'm obviously
overlooking something.

Please review kindly. I think adding a fuzzer like blitters-test would
be useful for validation of these changes. Presumably we would like to
verify that we can blit between all identical formats from 1bpp to
64bpp. Should the generic implementation fallback to converting the BLT
into an image composite operation?

Thanks,
-Chris