Tuplet brackets should reliably not follow kneed beams, regardless of whether Beam...

Tuplet brackets should reliably not follow kneed beams, regardless of whether Beam::is_knee has been called yet.

This affects input/regression/tuplet-number-kneed-beam-horizontal-fit,
and the following:
{
  \override TupletBracket.bracket-visibility = ##t
  \times 1/2 { c8 c''' c c''' }
}

Is it safe to call Beam::is_knee in Tuplet_bracket::calc_position_and_height?