feat: [WIP] use custom render objects for MarkerLayer

[JaffaKetchup]

No description provided.

[JaffaKetchup]

Current status:

• Improvement in UI thread with 12000 of new markers from 75ms to ~40ms
  • Not entirely sure why this occurs even on a single visible world
  • Haven't even implemented pre-build culling yet
• No improvement in Raster thread
  • It's likely what I've written is not very efficient

When not counter-rotating, I'm struggling to make different worlds still line up properly:

When counter-rotating, I haven't yet drawn each marker across multiple worlds:

@monsieurtanuki If you've got spare time and you're interested, maybe this is interesting to you - all this coordinate maths and rotations is difficult for me. But no worries if not ofc :D

[monsieurtanuki]

@JaffaKetchup I've just had a look at the code and it's too flutter-ish for me (e.g. MultiChildRenderObjectWidget), so I cannot help at that stage. I understand that it's still a WIP, but please make sure it's eventually readable enough to be maintainable.

If I remember well when I worked on osmdroid, I believe a simple way to deal with rotation would be to

• rotate the whole big widget - according to the center of the map widget and the rotation
• rotate the display of each marker in the opposite angle
• I think there's already a Rotate widget

About culling, I lazily considered the map diagonals and said that everything beyond the circle whose diameter is that diagonal is to be ignored (modulo the size of the widget).

In osmdroid we displayed the markers directly on the canvas. Could be a way to improve performances, who knows?

Hope this helps.