Set inter_tx_size for supertx coded blocks.

Set inter_tx_size for supertx coded blocks.

The loop filter relies on inter_tx_size in MB_MODE_INFO being set
properly when VAR_TX is enabled. Supertx coded blocks did not set this
previously at all, and the differing garbage values eventually resulted
in in a YUV mismatch between encoder and decoder after loop filtering.

This patch fixes this by setting inter_tx_size to the proper supertx
size in both the encoder and the decoder. This should also mean that
loop filtering is done at the proper transform boundaries, even when
supertx or vartx is being used.

Change-Id: I41a564cd6d34ce4a8313ad4efa89d905f5ead731