Eighth rest position dependent on other voice.

[Richard Giraud]

> I'm not top posting.

Please pardon if this has already been reported; 
I couldn't find mention of it in the bug tracker.

Expected Behavior:
When a rest follows a note, it appears on the 
same line as the preceding note. 

Observed Behavior:
The vertical position of the rest depends on 
notes in a different voice.  This occurs with
2.14.1 and 2.15.5.

Impact:
In some cases, the rest is pushed into the triplet 
bar for no good reason.

----

In the code below, the rest is where it's expected:

\version "2.15.5" 
\paper{ ragged-right=##t }
\new Staff <<
  \new Voice { \voiceOne {e''8 r} }
  \new Voice { \voiceTwo {a'4} }
>>

----

In the code below, the rest is higher than expected:

\version "2.15.5"  % Also occurs in 2.14.1.
\paper{ ragged-right=##t }
\new Staff <<
  \new Voice { \voiceOne {e''8 r} }
  \new Voice { \voiceTwo {c''4} }
>>

----

In the code below, the rest is where expected:

\version "2.15.5"  
\paper{ ragged-right=##t }
\new Staff <<
  \new Voice { \voiceOne {e''8 r} }
  \new Voice { \voiceTwo {c''8 r} }
>>

----

The code below demonstrates why this is annoying: the rest is moved up 
in a way that seems random 
and also hampers readability.

\version "2.15.5"  
\paper{ ragged-right=##t }

hhRhythm = \drummode {\repeat unfold 4 \times 2/3 { hh8[ r hh] } }

rhythmOne = \drummode {
    \override Beam #'damping = #+inf.0
    bd4 \times 2/3 { sn8[ r bd] } bd4 sn4
}

\score {
\new DrumStaff <<
  \override Score.SpacingSpanner #'spacing-increment = #5.5
  \new DrumVoice = "first" { \voiceOne << \hhRhythm >> }
  \new DrumVoice = "second" { \voiceTwo << \rhythmOne >> }
>>
}