Refactor canvas and loop closing logic

[almarklein]

This started with an attempt to clean up the wx closing mechanics, but ended up being a complete overhaul of the closing mechanics of the base class and all backends.

We have many more backend-specific tests that run on CI now, helping make rendercanvas more stable across the diffetent backends.

Changes

• All closing flows must go through BaseRenderCanvas.close().
• When the last canvas is closed, the loop is stopped. The base class handles this.
• When the loop is stopped, all canvases are closed.
• We detect loops starting with a little coroutine, but no longer poll canvases being closed with a loop-task.
• We detect loops closing either by 1) the last canvas being closed, or 2) for async loops by the coro to be cancelled.
• Added comprehensive tests for all backends, especially to test stopping and cleanup for GUI backends. All run on CI.
• The wx loop is now a fully functional loop (it was a bit of a special case before this pr).
• Wx canvas are now cleaned up by gc.
• For all backends, loop.run() without any canvases, does one flush of the event queue and then closes.

Notes for backend authors

TODO