Refactors and adds superres-mode 3 and associated
paramters --superres-qthresh and --superres-kf-qthresh
that are used to trigger superres mode when the qindex
for any frame exceeds the thresholds provided for non-key
and key-frames respenctively. The superres scale factor
numerator is progressively reduced from 16 starting from
that q threshold following a fixed slope.

Change-Id: If1c782993667a6fbaaa01bbde77c4924008c0d28