Adds variance based fixed size partitioning

Adds a method for determining a fixed size partition based on
variance of a 64x64 SB. This method is added to rtc speed 6.
Also fixes a bug in rtc_use_partition() and includes some
refactoring related to partitioning search, and some cosmetics.

Currently compared to speed 5, the coding efficiency of speed 6
is -19% and that of speed 7 is -55%, in cbr mode.

Change-Id: I057e04125a8b765906bb7d4bf7a36d1e575de7c6