GLTF Mesh Import: Support face splitting for faces with more than 65532 vertices

[akleshchev]

Collada importer splits meshes that don't fit into viewer's limits into multiple faces.

At the moment gltf import drop prims with a warning when prim.getVertexCount() is over limit.

Task:

For parity sake make GLTF importer split extra triangles into separate faces. See 'verts.size() >= 65532' check in load_face_from_dom_triangles

[aiaustin]

I think this enhancement if implemented would be very useful indeed.

The main issue I have been having with my glTF test meshes is finding (resolved now with a Blender python script that shows the highest vertex count mesh parts) and then sensibly breaking up mesh parts into < 64K vertex chunks. This is especially problematic after using a glTF transformation tool to reduce mesh part count before upload, which is essential in many cases. At this post Blender export stage its not a problem if mesh parts are mangled up as its not the "maintained" model. But the parts that get flattened, joined and welded really can be quite large and almost impossible to sensible split by the creator into 64K vertex chinks.