Web-mercator tiles don't touch the poles.

Tiles near the viewport center should be loaded first. This commit
fixes a discontinuity in the distance function that is used to sort the
queue of requested tiles.

crossbeam-utils needs Rust >= 1.25.0

by examining the stringtable for each PrimitiveBlock before iterating
over the contained elements.

* Add --keyval command line option to search for elements with a
  specific tag
* Add query module and Query trait to abstract over different types of
  queries

medium precision causes visible seams between tiles on some drivers.

which only includes a dummy implementation.

Add functions to convert screen coordinates to MapCoord/LatLonRad.

Partial solution. Check AABB of tile vertices.

Toggle with Ctrl+h

and restore the projection of the last session.

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

This enables the implementation of TileAtlas::textured_visible_tiles for
different kinds of visible tiles, e.g. for different layers.

and TileLayer -> MercatorTileLayer

This fixes a glutin/winit issue that resulted in 100% CPU usage because
of repeated X11 ConfigureWindow/ConfigureNotify events.

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

ViewMode::Flat -> Projection::Mercator
ViewMode::Globe -> Projection::Orthografic

Still not solved. Tried to approach by calculating properties of
elliptical arcs, but there are so many edge cases...

* Toggle between ViewMode::Flat and ViewMode::Globe with Ctrl+G
* Approach tile visibility problem at the sphere

This removes the holes of the globe at the poles.

A work in progress...