enable bitstream lossless support

1. Added a bit in frame header to  to indicate if a frame is encoded
in lossless mode, so decoder does not make the decision based on Q0
2. Minor changes to make sure that lossy coding works same as when
the lossless experiment is not enabled.
3. Renamed function pointers for transforms to be consistent, using
prefix fwd_txm and inv_txm for forward and inverse respectively

To encode in lossless mode, using "--lossless=1 --min-q=0 --max-q=0"
with vpxenc.

Change-Id: Ifae53b26d2ffbe378d707e29d96817b8a5e6c068