Improve CDEF parameter selection
Created by: tmatth
For a given frame, we can have up to 8 sets of CDEF parameters for superblocks to choose from (note: it is better to have fewer than 8 for lower bitrates, to reduce the cost of coding them per SB). Currently these are hard coded.
We could do a feedforward approach, where for the last frame of a given type, you greedily search for better CDEF parameters, then select those for the next frame of the same type (inter/intra, pyramid level, etc.?).
We could also look at stats and try and find decent values to hardcode for lower bitrates.
Once we can buffer a frame in rav1e, that would allow to do something closer to what libaom does, where we do multiple passes of one frame and greedily search for better sets, i.e. swapping in parameters that perform better.