[Pixman] NOOP implementation

[Søren Sandmann Pedersen]

The following patches add a "noop" implementation, which is used as
topmost in the implementation hierarchy. It is supposed to contain
iterators and compositing routines that don't do anything. For
example, there is a compositing fast path for the DST operator.

This is useful because it allows more CPU specific iterators to be
added without worrying about them being selected ahead of noop
iterators.


Soren