Separates the logic on transform type selection previously spread out
over a number of files into a separate function. Currently the tx_type
field in b_mode_info is not used, but still left in there to eventually
use for signaling the transform type in the bitstream.

Also, now for tx_type = DCT_DCT, the regular integer DCT is used, as
opposed to the floating point DCT used in conjuction with hybrid
transform.

Results change somewhat due to the transform change, but are within
reasonable limits. The hd/std-hd sets are slightly up, while derf/yt
are slightly down.

Change-Id: I5776840c2239ca2da31ca6cfd7fd1148dc5f9e0f