- Only warn about missing textures (but still emit the Texture directive
  for them.) Assume that the user has them / will want to replace them
  with some other texture
- Allow PNG textures now that we can read them
- Emit materials into a separate file. Always use NamedMaterials.
  (Both for ease of manual tweaking)
- Bugfix: don’t emit meshes without any indices.
- Don’t specify the Mitchell filter in the generated pbrt file