WIP: PlaneRegion extension, deployment, buffer work, and fix for #2360

[xiphmont]

This patch set is a multi-pronged approach at improving buffer usage in the loop filters by using a single consistent region interface (PlaneRegion), extending the PlaneRegion to add functionality previously only available from PlaneSlice while offering better boundary checking, eliminating a couple unneded copies/temp planes, and restoring the loop filter RDO loop to (mostly) Pixel<T> such that 8 bit assembly comes back into play along with reduced memory bandwidth pressure.

Ref: #2360