Keep track of MeshInstance and GeometryInstance override materials in the GLTFMesh object.
Ensure all arrays are non-empty to conform with "minItems":1 in glTF spec.
Fleshed out the "Optimize Mesh" options found in the mesh import UI
Gave a checkbox to every vertex attribute that can be compressed
Surfaced option to enable/disable Octahedral compression for
normal/tangent vectors
Also surfaces the vertex position compression option which previously
inaccessible because the defaults did not compress vertex positions
Supports all current importers (obj, fbx, collada, gltf)
For the time being we don't support writing a description for those, preferring
having all details in the method's description.
Using self-closing tags saves half the lines, and prevents contributors from
thinking that they should write the argument or return documentation there.
(cherry picked from commit 7adf4cc9b5)
