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