Handle SDEF and QDEF bone deformations

Godot only supports LBS (Linear Blend Skinning), which is BDEF1, BDEF2, and BDEF4 in MMD. MMD also supports dual quaternion skininng (QDEF) and spherical blend skinning (SDEF) in the same model.

I believe this can be implemented by storing some extra vertex parameters in CUSTOM arrays and using a custom shader for skinning.