Improve generated type hints and defaults

[almarklein]

• The logic in codegen that produces the method signatures is improved:
  • It produces type annotations without quotes.
  • The annotation is wrapped in Optional[..] if needed.
  • It takes the optional keyword in idl into account.
  • It no longer has None as a default value in cases where it should not.
  • It also generates type hints for methods that do not only consist of a descriptor.
• Can enable one lint rule that we disabled earlier.
  • TBH this was the reason I started this. Thought it was a quick fix, but ended up improving codegen for consistency with IDL quite a bit :)
• Apply check_struct() in a few more places, because the above fixes made the checker detect more cases.
• Enable making some args optional, even though IDL says they're not.
  • In practice this means that this PR does not introduce any backwards incompatible changes.
  • The cases where we apply this, include e.g. the depth_stencil arg in create_render_pipeline(). It's odd that IDL makes that field mandatory, because it means one cannot create a render pipeline without a depth-stencil attachment? We will have to look into this deeper when we implement the JS backend. It could well be an oversight in the IDL.