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

A work in progress...

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

Each Texture is bound to a unique texture unit (glBindTexture is called
only once). Context keeps track of the current active texture unit and
reduces the number of glActiveTexture calls to a minimum.

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.

Started 2015