Lots of small bug fixes, mainly around the transform size coding:

* The loop filter was accidentally using the non-subsampled
  block size for the V plane, due to comparing a plane index
  (0, 1, or 2) against PLANE_TYPE_UV (== 1)

* We allowed an initial update of the transform partition probabilities
  even on frames where we know they will never be used
  (because tx_mode != TX_MODE_SELECT).
  Further, these probabilities would not be reverted at the end
  of the frame, leading to the probability delta persisting across frames.

  Change this to behave more like the non-var-tx transform size coding,
  where probability deltas are only coded for frames with
  tx_mode == TX_MODE_SELECT, and the deltas only apply for one frame.

* Fix decoder for the case where the video as a whole isn't lossless,
  and we have tx_mode == TX_MODE_SELECT, but the current segment
  *is* lossless.
  Note that the encoder already does the right thing in this case.

* Don't allow the transform splitting to recurse "below" 4x4.
  This is really just a refactor, but means we can increase the
  maximum depth when subdividing rectangular transforms if we
  want to, whereas the previous code would have needed special cases
  for 4x8 and 8x4 transforms.

* Finally, when we hit the maximum splitting depth, don't update
  the counts as if we had coded a 'no split' symbol.

Change-Id: Iaebdacc9de81d2e93d3c49241e719bbc02e32682