added worker thread sleep throttling.

added worker thread sleep throttling.
each of the work object threads now self adjust their sleep interval based on the "fullness" of their fifo.
80% is the choose threshold.
Work objects with a fifo fullness of greater than 80% increase their sleep interval.
This allows other work object with less than 80% fullness more CPU usage.
Also adjusted thread_func, reader, and muxer sleep intervals to more reasonable values.

git-svn-id: svn://localhost/HandBrake/trunk@98 b64f7644-9d1e-0410-96f1-a4d463321fa5