changed conditions for extra voice start_x padding

In `getNoteStartX()` from `staves.js`, `start_x` gets increased by 20px if there is a `REPEAT_BEGIN` barline on the left. This is necessary when there are other start modifiers than barlines, but results in superfluous padding when additional modifiers are absent (this is often the case when the repeat barline doesn't occur at the beginning of a staff). I modified the 'multiple stave repeats test' to include a left `REPEAT_BEGIN` barline without modifiers and added a possible solution to the problem in `stave.js`.