Adds a new subpel motion function
Adds a new subpel motion estimation function that uses a 2-level tree-structured decision tree to eliminate redundant computations. It searches fewer points than iterative search (which can search the same point multiple times) but has the same quality roughly. This is made the default setting at speeds 0 and 1, while at speed 2 and above only a 1-level search is used. Also includes various cleanups for consistency and redundancy removal. Results: derf: +0.012% psnr stdhd: +0.09% psnr Speedup of about 2-3% Change-Id: Iedde4866f5475586dea0f0ba4cb7428fba24eee9