Increase coords consistency and clamp zoom level to numerically feasible range

[terrorfisch]

• Clamp the integer ZoomLevel to 0..31 when converted from floating point Zoom
• Restrict traversal of WorldTileCoords to supported zoom level range
• Construct QuadKey with the morton_encoding crate. This change is here because the previous QuadKey depended on the ZoomLevel type.

Fixes crash from #161 by making the "backend" more robust.

Does not implemented the desired zoom limit. This is done in #180

💻 Examples

You can arbitrarily zoom in/out in the demo without panic.

🚨 Test instructions

Launch demo and start zooming :)

✔️ PR Todo

I am not satisfied with these points yet and lack the domain knowledge to proceed:

• [ ] Check get_children usage in renderer
• [ ] Properly handle signedness of WorldTileCoords
• [ ] Check if saturation in ViewRegion::iter is the correct way to go