vmx: implement fast path composite_add_8888_8888

vmx: implement fast path composite_add_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              248.76          3284.48         +1220.34%
L2              264.09          2826.47         +970.27%
M               261.24          2405.06         +820.63%
HT              217.27          857.3           +294.58%
VT              213.78          980.09          +358.46%
R               176.61          442.95          +150.81%
RT              107.54          150.08          +39.56%
Kops/s          917             1125            +22.68%

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