GeometryShaderManager: Upload Line/Point width constants.
GeometryShaderGen: Redefine gl_InvocationID so we can use the same variable name in both backends.
GeometryShaderGen: Support multiple primitive types.

And make more stereoscopy code optional.
VideoCommon: Assume we always use a geometry shader, not just for stereoscopy.
VideoCommon: Merge LineGeometryShader into GeometryShaderGen.

This adds line-width emulation support to OpenGL.
VideoCommon: Merge PointGeometryShader into GeometryShaderGen.

This adds point-width emulation support to OpenGL.