MAX_NUM_TXB represents the maximum number of transform blocks in 1
dimension. CfL requires the maximum number of transform blocks in 2
dimensions. As such, we now use MAX_NUM_TXB_SQUARE instead of the
erroneous MAX_NUM_TXB for the average buffer.

There's an assert guarding for overrun on the average buffer.
This fix stops assertions on 4:4:4 sequences. This patch does
not alter AWCY results as AWCY only covers 4:2:0 sequences (for
which MAX_NUM_TXB turns out to be sufficient). 

Change-Id: I628db0131f60abc2d06cbbe3fe3dc40e28894ce3