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).
btw, I think it's fine to always generate this value but to only change it if stereo is enabled. I guess it ends in simplier code.