Always return a valid function from lookup_composite()

Always return a valid function from lookup_composite()

We never expect to fail to find the appropriate function as the
general_composite_rect should always match. So if somehow we fallthrough
the search, emit a _pixman_log_error() and return a dummy function.

Note that we remove some conditionals and a level of indentation hence a
large amount of code movement. This also reveals that in a few places we
are duplicating stack variables that can be eliminated later.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>