Web-mercator tiles don't touch the poles.

which only includes a dummy implementation.

Add functions to convert screen coordinates to MapCoord/LatLonRad.

Partial solution. Check AABB of tile vertices.

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This now uses an f64 transformation matrix (instead of f32) to transform
points on the sphere to screen coordinates. Conversion to f32 vertex
positions happens as a last step before uploading to a GPU buffer.

* Reach feature parity with mercator view:
  * Resize tile atlas to reduce draw calls
  * Fill in tiles from other zoom layers

Still a partial solution. Uses region growing starting from the central
tile and checks if at least one of the vertices is visible.

* Split up map_view module into
  * map_view
  * mercator_view
  * orthografic_view