VideoCommon: Merge LineGeometryShader into GeometryShaderGen.

This adds line-width emulation support to OpenGL.

### To-Do
- [ ] Disable the geometry shader stage if it is not needed.
- [ ] Check for geometry shader support in the API (OpenGL ES).
Where the hell did you find glPushAttrib / glPopAttrib? It's deprecated for a while and not supported on core or gles. In my opinion, it's also wrose to handle with this flag. Shall we just also emit the flipped primitive? We could also spend logic to curl them based on the culling settings, but I don't think this is worth.