OSM: Smooth navigation

[HuidaeCho]

• Tile caching
• Multithreaded downloads
• Partial zoom levels between integers using scaled tiles
• Zoom first using the current tiles and switch to the new image later (double buffering)

[HuidaeCho]

• https://stackoverflow.com/a/63305873/16079666
• https://stackoverflow.com/a/26703844/16079666
• https://wiki.tcl-lang.org/page/Tcl+event+loop

Tried multi-threading, but the canvas flicked too much. Also, it got stuck on thread.join() when I tried to cancel previous zoom events. Tkinter doesn't like threading. Used canvas.after() and canvas.after_cancel() to cancel subsequent zoom events in 9eced7c.

[HuidaeCho]

Partial zoom is impossible using tkinter's Canvas or PhotoImage. PhotoImage's zoom() and subsample() only take integer scales, and rescaling a canvas only rescales vector objects, not images. I tested both myself.

[HuidaeCho]

Zooming is fast enough on my machine, so double buffering is not really needed or its effect may not be noticeable on the dev machine. Anyway, I found that tkinter canvas is always double-buffered according to https://mail.python.org/pipermail/python-list/2001-March/090344.html.