Add layer position option for Merge Blend Tree

[iashbringer]

"Modular Avatar will create a new layer at the top of the FX controller. This layer will contain a single state, with Write Defaults on, and containing a Direct Blend Tree."

Some MMD worlds disable the first 3 layers of FX. I wish there was an option to place the resulting layer at the bottom of the FX.

[bdunderscore]

Ugh, talk about tech debt...

This one is a bit tricky to do anything about. Part of the goal of the merge direct blend tree component is to avoid a proliferation of separate layers; having an option to control position will make it more likely that there will be inconsistencies here, resulting in excess layers. Having it be a single option per avatar also might break down, as there are major differences in behavior when the blend trees are moved to different positions in the layer stack.

Those MMD worlds are already a bit broken anyway, as they're making a really big assumption that the first three FX layers just happen to control all facial expressions, which'll break down with things like FaceEmo. So I'm not really seeing any good solution here at the moment, unfortunately...

[iashbringer]

Can there be only top and bottom options?

[bdunderscore]

You mean - before and after the original FX layers in the avatar?

I'm not sure I want to set the precedent that things merged via Merge Animator are a bit of a second-class citizen there... but let me think about it a bit.