Design draft for 2D abstractions

**Summary**
In order to do 2D graphics (e.g. for creating UI controls) there should be an abstraction for 2D graphics in SharpGL. This abstraction should hide the complexity of OpenGL's 3D staff away (e.g. projection, third dimension, lighting etc.) and allow simple 2D drawing operations (e.g. Point, Line, Circle, Rectangle etc.).

**Acceptance Criteria**
1. There is a design draft that describes how the 2D abstraction looks like.
2. There is a first prototype for the 2D abstraction layer.