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).
No, because the geometry shader isn't always available so the Pixel shader can't always expect to get this value since the vertex shader doesn't supply it.