Fix #2678: Ungroup no longer multiplies the group's opacity into its members

Ungroup multiplied each member's opacity by the group's, so a member of a
50% group stayed at 50% after the group was gone, and members that
overlapped showed through each other. Each member now keeps its own
opacity, and the group's opacity is removed with the group.

crates/engine/src/cmd/object/arrange.rs

@@ -129,7 +129,7 @@ pub(super) fn ungroup(s: &mut Session, _: &Value) -> Result<Value> {
                 {
                     *clipping = false;
                 }
-                c.opacity *= n.opacity;
+                // Each member keeps its own opacity; the group's opacity goes with the group.
                 new_sel.push(c.id);
                 out.push(c);
             }