Collada animation layering

This MR adds animation layering for Collada format and with that AFAIK brings full animation support for Collada-animated games.

This doesn't do anything for Morrowind gameplay. With Vanilla Morrowind, there is probably a micro-slowdown at Animation::detectBlendMask (see changes in animation.cpp). This might add some modding options, but that's unexplored.

Later MRs related to Collada should likely be performance optimizations, quality improvements, and dehardcoding/moving bone name settings to a file other than settings.cfg.