Load multiple wimdo and camdo models

[ScanMountGoat]

Some models are split across multiple files. This applies to all supported games. One approach is to add model loading functions that take an iterator of paths. Loading a single model can call this function internally with std::iter::once. Merging a ModelRoot should also merge the skeletons. Skeleton merging can be tested with unit tests.

• load_models and load_models_legacy
• merge ModelRoot
• support multiple files for xc3_gltf and xc3_viewer

[ScanMountGoat]

It may be more accurate to load a single complete skeleton instead of trying to merge partial skeletons from wimdo skinning information.