fix #32441: bad layout with empty key signature

There are a number of layout issues that result when a measure contains an empty key signature segment, or even a key signature segment that contains no elements for the current staff.  This PR addresses the cases I could find, all of which are handled in different places in the code:

- space between clef and time signature
- space between clef and first normal chord / rest
- space between clef and mmrest symbol
- space between clef and full measure rest (also fixed the centering to work for multiple voices - both the ledger line on top voice, also the possibility of full measure rest in voice 2, which can happen currently after change in length to a voice 2 note in previous measure)