Support for shared contexts

Besides the interface to mark two (or more) contexts as shared, the registries needs to be split up to have both shared and unshared data.

Typically shared data:
- State of OpenGL Objects as
- Program
- Shader
- Texture
- Buffer
- ...
- Loaded Files
- Named strings (?)

Typically unshared data:
- State of compound OpenGL objects as
- Vertex Array
- Framebuffer
- OpenGL version (?)
- Implementation strategies