Support replacing animation clips in (nested) BlendTrees

[PiMaker]

A direct and 1D BlendTree can be used to combine multiple "basic material toggle" animations (e.g. for udim vertex discard) into a single layer, thus reducing layer count and optimizing performance - my AAV tool does this automatically for example (c.f. https://github.com/pimaker/animator-as-visual).

Previously, LazyOptimiser would break this setup for combined meshes because it wouldn't find BlendTree Motions and thus not replace any animation clips on them (even if they were found in the previous step).

Tested on my personal avatars that use AAV to generate their FX animators, both BlendTree and normal animations work fine after combining 2 SMRs.