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'm not talking about gl_Layer, I'm talking about this integer which is just called "layer". What's wrong with creating an output "flat out int layer = 0" in the vertex shader? :D