[font/draw] Move slanting to font layer I moved it to the draw layer in 11.0.0; That was a bad design: font-funcs should NOT be responsible for synthetic bold & slant. This design reverts that and makes the font layer apply it, using a recording. This is a minor API change from 11.0.0 since hb_draw_state_t::slant_xy which was introduced in 11.0.0, is removed now. I believe no client had started using it. It was only relevant to other font-funcs draw implementations.