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).
I actually did do that in an earlier D3D commit, but I didn't get feedback on it, anyway I know what to do now. It also doesn't matter if you miss `Apply()` you only need to apply the state when you're going to draw something.