Enable F821, and a few codegen improvements

[almarklein]

The flake8 F821 undefined name was ignored, because function annotations and pyshader.

• The IDL parser is now able to fully resolve types to either a Python builtin, or a GPU-class/flag/enum/struct.
• The codegen can therefore now generate annotations that are "valid".
• The structs module is therefore now also much cleaner.
• Where we use pyshader (in tests and examples) we still ignore this error.
• Improvement to codegen regarding ordering decorators and comments.