code: func (v *Context) MaskSurface(surface *Surface, surfaceX, surfaceY float64) {
  	C.cairo_mask_surface(v.native(), surface.native(), C.double(surfaceX),
  		C.double(surfaceY))
  }
docs: // TODO(jrick) Mask (depends on Pattern)
  // MaskSurface is a wrapper around cairo_mask_surface().