Rd thresholds change with block size.

Added structures to support independent rd thresholds
for different block sizes (and set experimental block
size correction factors).

Added structure to to allow dynamic adaptation of thresholds
per mode and per block size basis depending on how often
the mode/block size combination is seen (currently fixed factor).

Removed some unused variables.

TODO
- Adaptation of thresholds based on how often each mode chosen.
- The baseline mode values could also be adjusted based on
  the block size (e.g. for a particular intra mode use a low threshold
  for 4x4 prediction blocks but a relatively high value for 64x64.

Change-Id: Iddee65ff3324ee309815ae7c1c5a8584720e7568