This PR refactors the internals of the rendering engine, mainly the environment.py and the pipeline.py, mainly affecting how cache invalidation works.

The main purpose is to make it possible to use multiple different blend_mode settings in a single draw. This is a requirement for #492 e.g. to better support overlays.

For renderstate:

• Rename environment to renderstate.
• Refactor renderstate objects so the code is more clear.
  • In particular, create a renderstate object for each purpose,
  • And a combined renderstate object that wraps these.
• Refactor renderstate lifetime
  • The renderer takes responsibility for storing and clearing them.
  • Still support re-using these objects using a WeakValueDictionary.

For pipeline.py:

• Introduce object WeakAssociativeContainer to bind something to the lifetime of multiple objects at the same time.
• Use WeakAssociativeContainer to store pipeline-container-group objects on wobject+renderstate.
• PipelineContainer objects are now simpler because there is one for each specific wobject-renderstate combination, instead of having to manage differerent render states.