1. Algorithm modification:
Instead of having same filter threshold for a whole frame, now we
allow the thresholds to be adjusted for each macroblock. In current
implementation, to avoid excessive blur on background as reported
in issue480(http://code.google.com/p/webm/issues/detail?id=480), we
reduce the thresholds for skipped macroblocks.

2. SSE2 optimization:
As started in issue479(http://code.google.com/p/webm/issues/detail?id=479),
the filter calculation was adjusted for better performance. The c
code was also modified accordingly. This made the deblock filter
2x faster, and the decoder was 1.2x faster overall.

Next, the demacroblock filter will be modified similarly.

Change-Id: I05e54c3f580ccd427487d085096b3174f2ab7e86