Fix loading camera in gltf

[nelianos]

• Loading a .gltf/.glb with a camera, will currently not load the camera object to the Trimesh.Scene, and instead contain an empty "dangling" node that was originally the camera node.
• This is shown with the unit test; previously the loaded camera would have default values different than the original
• In this PR, we suggest loading the camera into the Scene object if it exists in the gltf, populating the gltf-compatible fields (fov, name, z_near) and the camera_transform

[mikedh]

Thanks for the fix looks great!