The var-tx has its own suite of tx size/type RD search functions,
which recursively split the partition into square tx blocks.

The Daala-dist requires access to 8x8 pixels (both decoded and predicted)
since it measures the distortion for multiple of a 8x8 pixels.
Thus, if tx block is smaller than 8x8, it waits until all of sub8x8 blocks
are RD searched (with MSE) then replaces the MSE of 8x8 pixels with
daala-dist's calculated distortion for 8x8 pixels.

It is also applied to luma pixels only.

Change-Id: Ic4891e89b4ef05cf880aa26781d2d06ccf3142de