Tweaks to examples

[almarklein]

• Refactor triangle.py and cube.py so that they return the draw function. This means that the canvas needs nothing more than WgpuCanvasInterface, which is nice and clean.
• The examples that use triangle or cube, call canvas.request_draw() or call the draw function on their own terms.
• The draw function returned by the async setup-method, is also async.
• Tweaked cube.py so it used buffer.map(), so we can run something that is actually async.
• Implemented a workaround for a bug that left destroyed glfw windows on screen on macos.

Side note

I observed the following warning when using glfw:

2024-10-02 16:13:41.233 python[40243:5114401] +[IMKClient subclass]: chose IMKClient_Legacy
2024-10-02 16:13:41.233 python[40243:5114401] +[IMKInputSession subclass]: chose IMKInputSession_Legacy

I checked that this happens inside glfw.poll_events and a quick Google suggests it is related to the latest MacOS Sequoia update. So nothing we can do on our end.