Move fill delegation into pixman-implementation.c

Move fill delegation into pixman-implementation.c

As in the blt commit, do the delegation in pixman-implementation.c
whenever the implementation fill returns FALSE instead of relying on
each implementation to do it by itself.

With this change there is no longer any reason for the implementations
to have one fill function that delegates and one that actually blits,
so consolidate those in the NEON, DSPr2, SSE2, and MMX
implementations.