Clef not printed when middleCPosition is changed

[Trevor Daniels]

There is an example in the User Manual, section 1.1.3,
Displaying Pitches, subsection Clef which shows a bug.  The
example is the one which shows the effect of changing the
three properties, clefGlyph, clefPosition and
middleCPosition.  Changing middleCPosition does not cause a
new clef to be printed, although subsequent notes are
correctly shifted.

Here's an example which shows the bug in 2.10.20, and as the
manual for 2.11.32 also shows the incorrect behaviour I
guess it is still present in 2.11.

{
 \clef "treble"
 c'4
 \set Staff.middleCPosition = #4
 c'4
 \set Staff.middleCPosition = #0
 c'4
}

Trevor Daniels