\alterBroken doesn't work for properties accessed before line breaking

[address@hidden]

If \alterBroken tries to set a property consulted before line breaking, it 
results in the property having an incorrect value:

For example...

\relative c'' {
  \alterBroken #'direction #'(UP DOWN) Staff.DynamicLineSpanner
  %\override Staff.DynamicLineSpanner.direction = #UP %% no issues
  a\< \repeat unfold 38 a a\p
}

This can be fixed by either:

a) Making it work for properties accessed in the original spanner (direction, 
padding ,etc.).
b) Changing the documentation to reflect that alterBroken only applies to 
properties that are accessed after line breaking.

Cheers,
MS