Refactor gm/wm/obmc for cleaner warping interactions

This creates a central function which defines when a
block should be warped. It also refactors the
WARPED_MOTION code so that all calls to av1_warp_plane
happen in the same location.

No change in performance.

Change-Id: Icaf9ec7700d34523809258594bb9843bb2975f46
10 files changed