This commit fixes an issue in source frame border extension. It
causes certain frame resolution such as 640x480 to have a portion
of the right/bottom extension filled by zeros, which misleads
motion search and degrades transform coding performance when large
block size is used.

This fix improves the speed 2 compression performance of a few
yt sequence, typically ranging from 1% - 2%, up to 5% at median
to low bit-rate.

Change-Id: Id6b09a5695d9e7651c6dfbc2c6a72288b08af7fb