test: safeguard the scaling-bench test against COW

test: safeguard the scaling-bench test against COW

The calloc call from pixman_image_create_bits may still
rely on http://en.wikipedia.org/wiki/Copy-on-write
Explicitly initializing the destination image results in
a more predictable behaviour.

V2:
 - allocate 16 bytes aligned buffer with aligned stride instead
   of delegating this to pixman_image_create_bits
 - use memset for the allocated buffer instead of pixman solid fill
 - repeat tests 3 times and select best results in order to filter
   out even more measurement noise