Overhauling the thresholds and mixing proportions for mfqe postprocessor.

Makes the thresholds for the multiframe quality enhancement module
depend on the difference between the base quantizers. Also modifies
the mixing function to weigh the current low quality frame less if
the difference in quantizer is large. With the above modifications
mfqe works well for both scalable patterns as well as low quality
key frames.

Change-Id: If24e94f63f3c292f939eea94f627e7ebfb27cb75