Improve SkinnedMeshRenderer culling by providing a better AABB

[atteneder]

At the moment culling problems are avoided by enabling Update When Offscreen setting of SkinnedMeshRenderers (SMR), which comes at a performance cost. This issue is about finding a better solution.

Solution 1: Enable Update When Offscreen (status quo)

• Pro: rapidly implemented
• Con: Performance regression as a lot of per-frame calculations are necessary

Solution 2: Assign a particular Transform as root bone

Figure out which bone/Transform has the most influence on a SMR and assign it as root bone (plus adjust/offset the bounds accordingly)

This works correctly if one (1) bone only influences the SMR. In glTF bone weights come in sets of 4, so the importer would have to iterate all influences to check it.

This won't scale well with vertex count.

Solution 3: Calculate AABBs that cover all movement over time

Given that each vertex's position can be influenced by many transforms that animate over time, calculating the AABB that covers all frames and positions is not trivial. Usually this is done on the GPU, so there's no way of retrieving that information easily.

Still, one could put in the work and calculate it, but I assume it will be:

• A lot of effort to implement
• A (one time) performance impact at loading time to do the AABB calculations (though that's not an issue for Editor Import)
• Results in bad culling behavior again if the motion is excessive and the bounds end up covering most of the space the camera moves in anyways.

These ideas initially came up in #301

[in0finite]

One idea would be to use SkinnedMeshRenderer.BakeMesh() API, which creates snapshot of the skinned mesh on the CPU. So, someone can compute AABB per frame, or per anim, and then assign it manually.

This could be used both at runtime (compute once and cache AABBs) or in edit-mode (prebuilding AABBs).