Improve typing

[almarklein]

With VSCode's Pylance set to "standard" I went through the "problems" it found.

• Applied small refactoring or a well-placed # type: ignore to make Pylance happy.
  • Resolved all problems in the "base" modules.
  • Did not touch the backend module much.
  • Examples should be without problems -> users who are strong on their type-checking should be able to use rendercanvas without getting type errors.
• Fixed one type error: get_physical_size() -> Tuple[int] should be Tuple[int,int].
• Same for get_logical_size().
• Found one case where canvas.__scheduler was used without checking it was None.