Refactor ext-intra

Miscellaneous changes. Mostly code simplification and cleanup.

Make luma max angle delta depend on block size, which is the
way it was before the nextgenv2 branch merge.

Also some fixes for compatibility of ext-intra and alt-intra.

Change-Id: I2ce869e6ba7dee765a8dc4802f5e88040db3df22
7 files changed