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).
I could set it to zero, but we don't really care about it since it is all multiplied by zero anyway.