Multiframe quality enhancement postprocessing

Adds a multiframe postprocessing module to enhance the quality of
certain frames that are coded at lower quality than preceding frames.
The module can be invoked from the commandline by use of the --mfqe
option, and will be most beneficial for enhancing the quality of
frames decoded using scalable patterns.

Uses the vp8_variance_var16x16 and vp8_variance_sad16x16 function
pointers to compute SAD and Variance of blocks.

Change-Id: Id73d2a6e3572d07f9f8e36bbce00a4fc5ffd8961