Refactor coefficient group scan mapping lists.

The relation between coefficients positions and coefficient group positions
was a big confusing due to the use of 16x16 diagonal coefficient mappings
also as coefficient group mappings.

- Moved all coefficient group mappings to their own const arrays and added
  a new array the select the correct coefficient group mapping. This removes
  special cases for 8x8 and 32x32 transform sizes.

- Removed all coefficient group mapping initialization from init_sig_last_scan.

- Removed 128x128 and 64x64 from regular coefficient group array as those
  transform sizes don't exist anymore in HEVC.