SSE2 optimization for nearest scaled over_8888_n_8888

SSE2 optimization for nearest scaled over_8888_n_8888

This operation shows up a little bit in some of the html5 based
games from http://www.kesiev.com/akihabara/

=== Cairo trace of the game intro animation for 'Legend of Sadness' ===

before:
[  0]    image    firefox-legend-of-sadness   46.286   46.298   0.01%    5/6

after:
[  0]    image    firefox-legend-of-sadness   45.088   45.102   0.04%    6/6

=== Microbenchmark (scaling ~2000x~2000 -> ~2000x~2000) ===

before:
    translucent: op=3, src=8888, mask=s dst=8888, speed=131.30 MPix/s
    transparent: op=3, src=8888, mask=s dst=8888, speed=132.38 MPix/s
    opaque:      op=3, src=8888, mask=s dst=8888, speed=167.90 MPix/s
after:
    translucent: op=3, src=8888, mask=s dst=8888, speed=301.93 MPix/s
    transparent: op=3, src=8888, mask=s dst=8888, speed=770.70 MPix/s
    opaque:      op=3, src=8888, mask=s dst=8888, speed=301.80 MPix/s