We are going to have several commits to setup new low/high
bitdepth data path selection logic. This patch is for inverse
transform. Let me summarize the ideas as following.

- For low/high bitdepth selection, encoder depends on
  input configuration, e.g., video sequence bitdepth,
  profile. Decoder depends on input bitstream. This has
  nothing to do with compiler/build  configuration.

- Typical encoder usage for sampling format 4:2:0.
  1) 8-bit video sequence:
   a) --profile=0
   Fastest encoding/decoding pipeline on speedup.

   b) --profile=2 --bit-depth=10
   Image pixels are left shifted by 2 bits. It
   employs 16-bit reference frame buffer and has high
   calculation precision. It usually enjoys higher
   compression performance.

  2) 10/12-bit video sequence (HDR):
   --profile=2 --bit-depth=10/12

- Transform coefficient type:
  Lowbitdepth:  int16_t
  Highbitdepth: int32_t

- The type, tran_low_t is still used in codebase,
  Which is int32_t, defining the data path capacity.
  Naturally, it is high bitdepth.

Eventually we shall remove the configuration flags,
CONFIG_HIGHBITDEPTH/CONFIG_LOWBITDEPTH, and seperate
low and high bitdepth data path. Two data paths co-exist
in the same build environment.

Change-Id: I35c06d4d4f19ebf80d909168fdddbae57c3cc884