Add wide source picture, external alpha, and transformed image routines.

Add wide source picture, external alpha, and transformed image routines.

The wide external alpha path should work correctly with wide formats.  The wide
transformed fetch code for now just does a 32-bit fetch and then expands, which
will lose precision.  Source pictures, for now, are evaluated at depth 32 and
then are expanded to depth 64.  We could get higher precision by evaluating them
directly at depth 64, but this should be good enough for now.

Signed-off-by: Søren Sandmann Pedersen <sandmann@redhat.com>