code: func (v *Context) SetSourceSurface(surface *Surface, x, y float64) {
  	C.cairo_set_source_surface(v.native(), surface.native(), C.double(x),
  		C.double(y))
  }
docs: // TODO(jrick) SetSource (depends on Pattern)
  // SetSourceSurface is a wrapper around cairo_set_source_surface().