Can now load multiple primitives
before we were at renderable = mesh = 1 gltf primitive
now we do rendrable = submesh = 1 gltfp primitive, with no real "mesh"
1 renerable = 1 VAO, 1 material, 1 texture binding set and 1 shader
TODO add a mesh abstraction that is a bunch of "renderables" that form
TODO do not load the same texture multiple times!