I should have also mentioned that the new signature is document in
backend_bases in the _draw_markers method

    def _draw_markers(self, gc, path, rgbFace, x, y, trans):
        This method is currently underscore hidden because the
        draw_markers method is being used as a sentinel for newstyle
        backend drawing

        path - a matplotlib.agg.path_storage instance
        Draw the marker specified in path with graphics context gc at
        each of the locations in arrays x and y. trans is a
        matplotlib.transforms.Transformation instance used to
        transform x and y to display coords. It consists of an
        optional nonlinear component and an affine. You can access
        these two components as

        if transform.need_nonlinear():
          x,y = transform.nonlinear_only_numerix(x, y)
        # the a,b,c,d,tx,ty affine which transforms x and y
        vec6 = transform.as_vec6_val()
        ...backend dependent affine...