MFQE: apply threshold to subblocks and chroma.
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
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