Improvements to the api regarding skinning-style routines

[OptimisticPeach]

This is following a discussion we had here on discord. A brief summary of my usecase and the issues I ran up against:

• My "skinning-style routine" is projecting points from 4D to 3D.
• Meshes hence have source Vec4 positions. These get their own "4d position" vertex attribute.
• The output vertices are Vec3 positions.
• The way the skinning routine works currently, the skinned vertices get taken and placed from Vec3 buffers. Hence, meshes which are skinned have the "vec3 position" vertex attribute.
• My mesh does not have a "vec3 position" attribute -- hence it would be rejected by this code (snippet in case the link gets invalidated):

  ...
  for &required_attribute in M::required_attributes().into_iter() {
      // We can just directly use the internal mesh, as every attribute in the skeleton is also in the mesh.
      let found_in_mesh = args
      ...
  

• Hence my solution is to add the "3d position" and "3d normal" attributes to my mesh and fill them with dummy values.
  • Add a way to add vertex attributes without uploading data for them.
• This wouldn't let have multiple objects with the same particular mesh instantiated, since this buffer would be created per-mesh, not per-object.
  • Add a way to have multiple output buffers per mesh.

I hope that summarizes the conversation.