Make SB coding size-independent.

Merge sb32x32 and sb64x64 functions; allow for rectangular sizes. Code
gives identical encoder results before and after. There are a few
macros for rectangular block sizes under the sbsegment experiment; this
experiment is not yet functional and should not yet be used.

Change-Id: I71f93b5d2a1596e99a6f01f29c3f0a456694d728