WIP: PlaneRegion extension, deployment, buffer work, and fix for #2360#2390

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xiphmont wants to merge 4 commits intoxiph:masterfrom xiphmont:work_on_2360_b

+692-408

Commits on Jun 17, 2020

First steps to removing unneeded hardwiring of u16 in the rdo_loop_decision pipeline
xiphmont
committed
Eliminate a redundant copy/buffer in loop filter RDO
xiphmont
committed

Commits on Jun 19, 2020

Swap usage of PlaneSlice for PlaneRegion in loop filter, add superregions
xiphmont
committed

Commits on Jun 23, 2020

Implement fully bounds-checked pad and visible area access
xiphmont
committed