Refactor skinned mesh weight data structure

[appgurueu]

This is in preparation for GPU skinning. It's not helpful to have weights associated with joints, we need vec4's of weights and joint IDs as vertex attributes which we can upload to the GPU as a UBO (on OGL (ES) 3 drivers, that is).

A neat side effect is that we can get rid of the ugly Vertices_Moved.

How to test

Make sure that all your favorite skeletal animation regression tests (e.g. the ones from devtest) don't do anything funky.

[lhofhansl]

I tried. And it seems to work - at least with all models I tried.
The change is a lot to review, though.

[appgurueu]

Alright, made more changes based on the comments and some more things that bothered me, I think it's pretty neat now.

Unfortunately that meant I had to increase the diff by a bit, but it should be at least nicely split up now.

[SmallJoker]

Whoa that's a lot of code.... second (static) review.
I'll try to do some tests with this PR on servers in the next few days.

[appgurueu]

Thanks for taking a look! I think I have addressed everything now.

[SmallJoker]

No issues found with:

☑️ Lava flan
☑️ Several animated mobs on a server
☑️ Sam
☑️ Animated spooder

Works on my machine.

[lhofhansl]

And I tried with Draconis and NSSM. Works fine.
It's a lot of code... I read through it and it all looks reasonable.