frame_superres: Add experimental flag
frame_superres: Add experimental flag

Frame Super-Resolution is an experiment currently under development.
This commit just adds an experimental flag for it.

The goal of the experiment is to compress a downsampled version of a
frame instead of the full resolution one when bandwidth is limited. The
decoder will need to upsample and filter the frame it recieves after
decoding. It is thought that this will result in a visually superior
result in low bandwidth scenarios.

Change-Id: Ia8af8692cffd133db3838693fd85ded2442b078c

GrokImageCompression/aom,luctrudeau/aom,smarter/aom,smarter/aom,luctrudeau/aom,GrokImageCompression/aom,luctrudeau/aom,smarter/aom,smarter/aom,luctrudeau/aom,GrokImageCompression/aom,smarter/aom,smarter/aom,GrokImageCompression/aom,GrokImageCompression/aom,luctrudeau/aom,luctrudeau/aom,GrokImageCompression/aom