Save the final build of mask
Save the final build of mask

Use a temp buffer to save the built mask, and copy
from this temp buffer if necessary.

For encoder, about 0.2% faster shows by encoding
20 frame of BasketballDrill_832x480_50.y4m at 800kbps
on speed 1 with no performance change.
( 212664 ms -> 212103 ms)

Change-Id: I7a321c9c79db41ce088e9701cf039c3c7891dc48

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom