[Feature] DynamicEnvironmentMapManager optimizations

[mzschwartz5]

Feature

Right now, a DynamicEnvironmentMapManager (DEMM) is created per-model or per-tileset to facilitate more realistic PBR lighting.

We've had some complaints that, for scenes with many models colocated, DEMMs can cause a big performance hit - and even some bugs where not all models update their dynamic lighting. For such scenes, it might not even be necessary to have a DEMM per model. For example, in a scene with a forest of trees, a single DEMM might be sufficient for all models.

Perhaps we can cache and reuse DEMMs at either the scene level, or per globe surface tile. The latter sounds appealing to me, because DEMMs could then benefit from the refinement that surface tiles do automatically.