Fairly basic integration of rectangular blocks in encoding RD loop.

Adds RD integration for 32x16, 16x32, 64x32 and 32x64 rectangular blocks.
Derf almost +0.6%, HD a little over +1.0%, STDHD +1.3%.

Change-Id: Id651fdb6a655fdbb5c47009757e63317acfb88a5