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

This would reduce the calls to glUseProgram if there actually was more
than one 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.

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

Started 2015