In cases where you have a flat background occluded by a moving object
of similar luminosity in the foreground, it was likely that the
foreground blocks would persist for a few frames after the background
is uncovered. This is particularly noticable when the object has a
different color than the background, so add the chroma planes in as an
additional check.

In addition, for block sizes of 8 and 16, the luma threshold is
applied on four subblocks independently, which helps when only part of
the background in the block has been uncovered.

This fixes issue #392, which includes a test clip to reproduce the
issue.

BUG=392

Change-Id: I2bd7b2b0e25e912dcac342e5ad6e8914f5afd302