Compute src, mask, dest flags and base fast path decisions on them.

Compute src, mask, dest flags and base fast path decisions on them.

This makes sets the stage for caching the information by image instead
of computing it on each composite invocation.

This patch also computes format codes for images such as PIXMAN_solid,
so that we can no longer end up in the situation that a fast path is
selected for a 1x1 solid image, when that fast path doesn't actually
understand repeating.