Improve terrain subdivision and raster tiles fecthing

[jailln]

Context

Several bugs are related to terrain subdivision and raster tiles fetching - for instance #2159 ; #2214 and the following behaviour that can sometimes happen (unfortunately I can't share the terrain source for this one for now):

output.mp4

The terrain subdivision code (mostly in TiledGeometryLayer) has become quite complex, is hard to understand (see discussions in #2159 and #2344 for instance) and is buggy (see aforementioned issues).

Proposed Solution

I think we need to:

• Refactor and simplify this part of the code that has a high cognitive complexity
• Extract and document the algorithms that control tile subdivision and raster tiles fetching
• That should eventually facilitate related bug fixing

I've started to work on a refactoring and simplification on this branch

[airnez]

Also related to #2442