First modification of multi-thread decoder

This is the first modification of VP8 multi-thread decoder, which uses
same threads to decode macroblocks and then do loopfiltering for each
frame.

Inspired by Rob Clark, synchronization was done on every 8 macroblocks
instead of every macroblock to reduce lock contention.

Comparing with the original code, this implementation gave about 15%-
20% performance gain while decoding my test clips on a Core2 Quad
platform (Linux).

The work is not done yet.

Test on other platforms are needed.

Change-Id: Ice9ddb0b511af1359b9f71e65066143c04fef3b5