cogl: Use SSE2 when possible for premultiplying

cogl: Use SSE2 when possible for premultiplying

This adds a fast path for premultiplying an RGBA image using SSE2
instructions. SSE registers are 128-bit and we need at least 16-bits
per component for the intermediate result of the multiplication so we
can do two pixels in parallel with one register. The function
interleaves 2 SSE registers to multiply 4 pixels in one function call
with the hope that this will pipeline better.

http://bugzilla.openedhand.com/show_bug.cgi?id=1939
Signed-off-by: Emmanuele Bassi <ebassi@linux.intel.com>