* lily/score.cc (Score): unprotect copy of Output_def. Plugs

* lily/score.cc (Score): unprotect copy of Output_def. Plugs
memory leak.

* lily/parser.yy (score_body): don't clone Score, that's done in
Lily_lexer::try_special_identifiers(). This fixes a memory leak.

* lily/score-engraver.cc (derived_mark): new function.
(initialize): unprotect Paper_score. This fixes a memory leak.

* lily/score-performer.cc (derived_mark): new function.

* lily/score-translator.cc (get_output): use SCM for get_output() call.