Earlier, intra prediction for rectangular blocks was performed by
running two steps of prediction on square sub-blocks.

With this experiment, we do proper intra prediction for rectangular
blocks. This ensures that we make use of all available neighboring
pixels especially for directional modes. For this, all the intra
predictors were updated to work with rectangular transform block sizes.

Performance improvements are small but free of cost:

All Intra frames:
lowres: -0.126
midres: -0.154

Video Overall:
lowres: -0.043
midres: -0.100

[Could not get AWCY results due to a backlog.]

BUG=aomedia:551

Change-Id: I7936e91b171d5c246cb0a4ea470a981a013892e6