Gets vertical skylines from grob stencils

input/regression/tuplet-number-outside-staff-positioning.ly

+\version "2.17.0"
+
+\header {
+
+  texidoc = "Grobs whose parents have @code{outside-staff-priority} set
+should figure into the vertical skyline of the @code{VerticalAxisGroup}
+such that grobs with a higher @code{outside-staff-priority} are correctly
+positioned above them.
+"
+
+}
+
+\relative c'' {
+   \override TupletBracket #'outside-staff-priority = #1
+   \override TupletNumber #'font-size = #5
+   \times 2/3 { a4\trill a\trill^"foo" a\trill }
+}