Support for skinning information in content pipeline

This is some progress I've made today getting skinned models through the content pipeline, and some other enhancements made along the way.

- OpenAssetImporter was missing some bones in its skeleton, trying another technique instead
- Added a TypeConverter for Vertex4 -> Single data.
- Implemented MeshHelper.FlattenSkeleton
- CompiledEffectContentWriter now correctly writes its reader type as EffectReader
- Enhanced some MGCB error messages with file hints where necessary