vmx: implement fast path composite_over_8888_8888

vmx: implement fast path composite_over_8888_8888

Copied impl. from sse2 file and edited to use vmx functions

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

POWER8, 16 cores, 3.4GHz, ppc64le :

reference memcpy speed = 27036.4MB/s (6759.1MP/s for 32bpp fills)

                Before           After           Change
              ---------------------------------------------
L1              129.47          1054.62         +714.57%
L2              138.31          1011.02         +630.98%
M               139.99          1008.65         +620.52%
HT              122.11          468.45          +283.63%
VT              121.06          532.21          +339.62%
R               108.48          240.5           +121.70%
RT              77.87           116.7           +49.87%
Kops/s          758             981             +29.42%

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