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).
SetConstants if much often called than SetXXXChanged. So I think it's faster to set all of those constants values direct in the SetXXXChanged function instead of checking for boolen values.