Move functionality to orthografic_view and mercator_view.

* 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

Toggle with Ctrl+h

and restore the projection of the last session.

and TileLayer -> MercatorTileLayer

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

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

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

A work in progress...

* Decode/search in parallel
* Start decoding the first Blob as soon as possible
* Update multiple markers at once

Use the parameters -s and --pbf for this.

* Cache the enabled vertex attributes in `Context`
* The vertex attribute parameters (offset, stride, ...) are not cached,
  yet
* Store parameters as `VertexAttribParams` in `Program`

Program, Buffer, Texture... structs no longer store a reference to a
Context. Now it has to be passed explicitly to all methods that mutate
the OpenGL state. This enables caching the state and removing redundant
calls to gl* functions.

and only call glViewport once per frame when resizing.

This should improve platform independence.

...instead of using the `description` method.