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 know, but I want to decide how to refactor it first, I think we should move the `SetGeometryShader()` to `g_renderer->ApplyState()`, but should we then also add `SetGeometryShader(nullptr)` to `g_renderer->RestoreState()`?