fix: 2080 - new "world safe" visible bounds for camera

[monsieurtanuki]

What

• This PR introduces the concept of "world safe" latlng bounds: with a north, a south, a longitude center and a longitude width.
• With "world safe" bounds, we can have bounds wider than a world.
• The bounds of the camera are computed as wide as they actually are. Therefore, potentially wider than the world, instead of limited to the currently projected left and right sides of the page.
• A typical use-case is for the camera bounds when we're culling polygons. We now compare polygon bounds with more relevant camera bounds.
• TODO:
  • try to refactor so that polygons and polylines share significant parts of the culling algorithm
  • try to refactor as there's repeated code about poly continuity, as a list of pixels and as a list of latlngs
  • maybe copy/move the concept of "drawInSingleWorld" down to the polygon or polyline level

Screenshots

one world	the other world

Impacted files

• camera.dart: now computes "worldSafe" visible bounds
• latlng_bounds.dart: new "worldSafe" constructor, that can be wider than a world, and longitudeCenter and longitudeWidth concepts. Refactored "isOverlapping" accordingly.
• polygon.dart: added widgets for the test