During the import process, many importer nodes are replaced with their
engine node counterparts. For example, ImporterMeshInstance3D is
replaced with a MeshInstance3D node. Any meta data set on these
importer nodes, i.e. through a GLTFDocumentExtension, are lost during
the conversion. This change copies over any meta data set on these
importer nodes to their engine counterparts.
Reimplements "Remove Immutable" by comparing to the skeleton rest.
It is necessary to delay removing animation tracks until after the correct rest pose is calculated in rest-fixer.
Preserves the original implementation in the GLTFDocument / FBXDocument API for compatibility.
This avoids using convex decomposition every time collision is enabled
on a mesh, which can be extremely slow on complex meshes (such as entire
levels).
