Doesn't this shift _everything_ that one might try to align relative to a clef, not just rehearsal marks?
What about key signatures? The break-align-symbols for RehearsalMark are (staff-bar key-signature clef). This change won't do anything for the alignment of a rehearsal mark that attaches itself to a key signature. I think it would be better for a RehearsalMark to be center-aligned at a bar line, but left-aligned at a key signature or clef (maybe generalized to anything with break-align-anchor-alignment = RIGHT?). I don't know if it's currently possible to define that, but it's probably worth thinking through what it would require. https://codereview.appspot.com/553290043/
