Add frame level buffer to store txfm coeffs

Doing tokenize in the last step of RD loop and then doing packing
tokens in bitstream packing phase is hard for debugging.

Therefore, we create a frame-level buffer to store the txfm coeffs
from the reconstruction in RD loop and then in bitstream packing
phase, we can code the txfm coeffs directly.

Change-Id: I999470eef6e038317a91585df2bdfc20aca3573e