Unexpected "Merge_rests_engraver" behavior

[Павел Буданов]

See the vertical position of multimeasure rest.

```lilypond
\version "2.19.83"
\new Staff \with { \consists "Merge_rests_engraver" } <<
   { R1 r1 } \\
   { R1 r1 }
>>
```