Handle OBJ groups (g) to divide an OBJ to multiple meshes automatically.

[vkoskiv]

Right now if you export a scene from Blender as an OBJ, it's all stuffed into a single mesh in C-ray, which completely breaks the k-d tree acceleration.
Whenever a g tag is encountered in an OBJ file, we should start loading a new mesh instead of adding to the current one.