vmx: implement fast path scaled nearest vmx_8888_8888_OVER

vmx: implement fast path scaled nearest vmx_8888_8888_OVER

It was benchmarked against commid id 2be523b from pixman/master

POWER8, 8 cores, 3.4GHz, RHEL 7.1 ppc64le.
reference memcpy speed = 24764.8MB/s (6191.2MP/s for 32bpp fills)

                Before           After           Change
              ---------------------------------------------
L1              134.36          181.68          +35.22%
L2              135.07          180.67          +33.76%
M               134.6           180.51          +34.11%
HT              121.77          128.79          +5.76%
VT              120.49          145.07          +20.40%
R               93.83           102.3           +9.03%
RT              50.82           46.93           -7.65%
Kops/s          448             422             -5.80%

cairo trimmed benchmarks :

Speedups
========
t-firefox-asteroids  533.92 -> 497.92 :  1.07x
    t-midori-zoomed  692.98 -> 651.24 :  1.06x

Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
Acked-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>