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).
yeah, remove it here. It isn't used in the PixelShaderManager, and the VertexShaderManager require an additional cleanup ...