VideoCommon: Merge LineGeometryShader into GeometryShaderGen.

This adds line-width emulation support to OpenGL, with stereoscopy support to boot!

### To-Do
- [x] Disable the geometry shader stage if it is not needed.
- [x] Check for geometry shader support in the API (OpenGL ES).
This is used in every shader cache, do you want me to change it for the VS and PS caches too?