[paint] Optimize PAINT_COMPOSITE

At the start of each paint call the current group is clear.
So we don't need to start a new group for the backdrop paint.

A paint composite really needs one group push, not two.

src/OT/Color/COLR/COLR.hh

@@ -1245,12 +1245,10 @@ struct PaintComposite
 
   void paint_glyph (hb_paint_context_t *c) const
   {
-    c->funcs->push_group (c->data);
     c->recurse (this+backdrop);
     c->funcs->push_group (c->data);
     c->recurse (this+src);
     c->funcs->pop_group (c->data, (hb_paint_composite_mode_t) (int) mode);
-    c->funcs->pop_group (c->data, HB_PAINT_COMPOSITE_MODE_SRC_OVER);
   }
 
   HBUINT8		format; /* format = 32 */

src/hb-ft-colr.hh

@@ -392,12 +392,10 @@ _hb_ft_paint (hb_ft_paint_context_t *c,
     break;
     case FT_COLR_PAINTFORMAT_COMPOSITE:
     {
-      c->funcs->push_group (c->data);
       c->recurse (paint.u.composite.backdrop_paint);
       c->funcs->push_group (c->data);
       c->recurse (paint.u.composite.source_paint);
       c->funcs->pop_group (c->data, _hb_ft_paint_composite_mode (paint.u.composite.composite_mode));
-      c->funcs->pop_group (c->data, HB_PAINT_COMPOSITE_MODE_SRC_OVER);
     }
     break;