This addresses an inconsistency between the set used
to decode the tx_type in the bitstream and the set used
for the tx_type search. Previously, the set used to
read/write the tx_type was based on the smallest tx_size
in the vartx partitioning, but the search uses a set
based on the largest possible tx_size. This patch
changes the tx_type search to use the transform type
set associated with the tx_size 1 recursive level down from
the max square tx_size to make the search more consistent
with the bitstream syntax. If a tx_size is selected for an
invalid tx_type, DCT_DCT is used for that partition instead.

This patch also adds assertions to all exposed transform
functions to ensure that no illegal transform type/size
combinations occur.

This currently gets a 0.1% drop in performance on lowres.
The drop is due to the reduction of the tx_types available
for 32x16 and 16x32 transform sizes. Before this patch,
32x16 and 16x32 transforms were getting assigned a
set of 12 tx_types, some of which we did not intend to
support for these sizes.

Change-Id: I44aca4876b261c345623cd04ad6235bca4532701