Smart tile iteration

[Maximkaaa]

Current implementation of TileScheme::iter_tiles has some limitations:

1. Tiles are iterated over rectangular are with constant resolution. This means that when the map view is tilted over x-axis, far away tiles are displayed with too high resolution, and we need to load extra tiles that are not actually displayed on the current view.
2. It always returns None if the map view Crs is not the same as the tile scheme Crs.

We need to find a smarter way to iterate over tiles. For starter, MapView::position and MapView::resolution are always the values at the center of the screen (map). So we can start from the center and go out to the sides, checking for every tile if it's still in the view.