Search and pick filter level for each superblock after the entire
frame has been encoded. And then apply loop filtering using
selected filter levels.

Now it can't work with LOOPFILTER_LEVEL. Y, U, V planes share the
same filter level.

Per superblock, filter levels are signaled via bitstream.

Future work: move selecting filter level into each superblock
rather than wait for the whole frame encoded.

Change-Id: I46cb994ec4866abb0456146f7c3cae8813c1d52e