More efficient Buffer and Texture updates

[almarklein]

Closes #321

Todo

• Introduce a registry that is aware of all buffers and textures, and which ones need an update (uses weakrefs).
• Make the renderer use that instead of checking all resources whether they need to be synced.
• Removed a bit of wgpu-specific logic from lights.py
• Updated the list in Logic in materials is based on details about the wgpu renderer #272 along the way.
• Removed a print-call from the fly controller.

How it now works

(also added this to the code in a comment).

• When a resource is first created, it's _wgpu_object attribute is None
  and its _wgpu_flags is unset.
• When the resource is actually being used somewhere, it will end up
  in the logic that creates a pipeline object (e.g. in _pipeline.py), which
  sets the appropriate usage flags (because that code knows how the resource
  is used) and then uses ensure_wgpu_object to create the object.
• Resources that need to be synced are tracked in the resource_registry,
  but only go into it when they have their _wgpu_object set (i.e. when
  the resource actually exists on the GPU).
• Right before the renderer performs a draw, it queries the registry
  and calls update_resource on each.

This PR basically makes the above work, and then removes the old system (which required updates more spread out over the code).

[almarklein]

About done. Two validation examples fail. Will look into that, but otherwise this is good for review.