Added a basic Rast module to the generic device and GL backend.
Improved the primitive state, fully implemented on GL backend
Depth test implemented in the GL back-end
Moved DrawState into the render module and exposed to the user.
Added cull & depth helpers for DrawState
Added Blend state types.
Blend mode setup implemented on the GL back-end
Implemented basic blend presets.
Fixed face culling, enabling by default now.
Merge pull request #79 from kvark/rast

Basic rasterizer states