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).
The d3d backend uses a caching state tracker. So please don't use flipping states, just disable the geometry shader everywhere else. While you're on grepping for Draw calls, does some of them miss the "D3D::stateman->Apply();" line?