Vertical alignment of successive trill spanner

[Daniel Rosen]

> I'm not top posting.

In the example below
(http://www.lilypond.org/doc/v2.16/Documentation/af/lily-df4bb625.ly
with three trill spanners instead of two), the spanners don't line up
on the same baseline, even though the NR (http://bit.ly/SRQG0Z)
says that "successive trill spanners will automatically become the right
bound of the previous trill."

\version "2.16.1"
\relative c' {
  d1\startTrillSpan
  d1\startTrillSpan
  b1\startTrillSpan
  d2\stopTrillSpan
  r2
}