Preliminary code for variance based paritioning

Brings back most of Jim's previous patch for choosing
partitioning based on variance while making it compatible
with the current state of the code. Also adds a
nonrd_use_partition() function to recursively encode  for any
arbitrary sb_type decisions within a 64x64 block; and
includes some refactoring.

Currently, when the VAR_BASED_PARTITIONING mode is turned on
for speed 7, there is a 10+% speed-up observed.

Experiments/improvements with this new partitioning method
will be conducted subsequently.

Change-Id: Ie6f43bfbde30583e941f450bf07c3b48828c9571