Refactor elevation data processing to show 3d terrain model, hill shading, and contour lines of any density

[biodranik]

This is a huge and complex task. Let's collect here useful references on how to:

• Efficiently approximate and compress elevation data so it won't take a lot of space
• Calculate the elevation at any coordinate in the fastest way
• Render 3d terrain in a fast way from the same elevation data

Other open-source projects have already implemented this so some ideas can be taken from there (e.g. OSMand, what else?)

This task depends on/is related to:

• [ ] https://github.com/organicmaps/organicmaps/issues/4163
• [ ] https://github.com/organicmaps/organicmaps/issues/8672

[pastk]

All maps apps that I know (OsmAnd included) use separate files for contour lines and hill shading / 3d model (and sometimes slopes).

[pastk]

Transformation between polygons (3D model) and contour lines seems to be quite resource-intensive which is probably why nobody does it on-device.