Correct displacement of stavenote keys on same line

This fixes an issue where two keys of a stavenote that are on the same line aren't consistently rendered with the correct displacement. 

*Example:* if F/4 and F#/4 are keys of a stavenote, only the first one is rendered. The second one is masked by the first. The expected behavior is that after the first one is rendered, the second one should be displaced and shifted to the right so that it is visible as well.

- [ ] add unit tests