Add ALT_INTRA experiment.
When the experiment is ON, we use Paeth predictor instead of TM predictor. For derf set, this gives about 0.09% improvement overall, and 0.55% improvement if all frames are forced to be intra-only. Also, if the EXT_INTRA experiment is also on, the improvement overall is 0.056%, and improvement if all frames are forced to be intra-only is 0.465%. Change-Id: Id74e107ede70a8d2107fa14fcb3f44b23a437274