Provided a path for building MeshDescriptions for static meshes at runtime, and supporting API.
Clarified use of attributes for static meshes; now some attributes may be considered optional.
Added FStaticMeshAttributes, used for fetching and registering all the attributes required by a static mesh.
Added Blueprint interface for building meshes.
Deprecated a number of existing APIs:
- FMeshDescription methods which referred explicitly to static mesh specific attributes
- StaticMesh attribute registration has been moved to FStaticMeshAttributes
#rb none
[CL 9123037 by Richard TalbotWatkin in Dev-Editor branch]
