This is clearly meant to be a fast path, but it appears to be a net
loss at this point.

If you really wanted to ricer-tune here, note that the inner loop of
fbBltOne is a bit too complicated for gcc (at least 4.8.2), it doesn't
specialize the loop for the handful of legal values of bitsPerDst, which
means computing at runtime what could have been const-propped.

Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Adam Jackson <ajax@redhat.com>