Reorder chroma coefficient generation.
Move scan order selection to a function.
Move chroma coefficient generation to it's own function.

- It's time to chop up this monster that is encode_transform_tree.
Separate luma and chroma coefficient generation variables.
Remove redundant variables.

- Redefine LCU_WIDTH to be 64. Stuff will break horribly if it's
  anything else anyway.

- Add LCU_WIDTH_C for chroma LCU width. It should be more readable than the
  constant (LCU_WIDTH >> 1).
Move transform skip decision to it's own function.
Move coefficient generation functions from encoder.c to transform.c.

- These functions probably should have been there to begin with.
Merge branch 'coeff_cleanup'