Refactor selected partition size coding for rtc

This commit makes a refactoring of the rtc_use_partition. It allows
the encoder to take a preferred block size for non-RD mode decision.
The boundary blocks are handled such that smaller block sizes that
fit in the boundary size will be used instread.

In rtc mode, the coding performance of speed -6 for pedestrian_1080p
goes from
158980 b/f, 38.934 dB, 22721 ms to
159008 b/f, 40.064 dB, 23721 ms.

For rtc set, the speed -6 compression performance is improved by
26%. Still about 2dB behind speed -5 at this point.

Change-Id: If0944f0880eaf1ad340bc325d97cea8d0f9dd53f