Optimise plane-suitability calculation in DRM backend

At the moment, for every view, we spend a great deal of time building and sorting a per-plane list, then filtering it, then for every plane doing suitability checks to make sure we can place it.

A lot of this can be cached and coalesced, so we can spend less time doing pointless individual tests. As a result, it makes the code slightly easier to follow.

Part of the goal of this reorganisation is to spread paint nodes wider, so we can reuse + cache + cull them.