Division by zero crash when window is too small
When the window becomes smaller than the `init`ed screen size, pyxel fails with
```
pyo3_runtime.PanicException: attempt to divide by zero
```

This happens due to `screen_scale`:
https://github.com/kitao/pyxel/blob/0784e5fa2c0a85d2571509ead9d86c55654aa2e0/lib/engine/src/sdl2.rs#L395-L399
Here, `screen_scale` can be `0` when there is not enough window space vertically or horizontally.

This then causes the division by zero here:
https://github.com/kitao/pyxel/blob/0784e5fa2c0a85d2571509ead9d86c55654aa2e0/lib/engine/src/sdl2.rs#L413-L414

If its relevant, I'm on Linux, though I doubt this is platform specific.

Ideally when there isnt enough space, the game view inside the window would shrink to fit the window size. 