fix #32231: width of measures with key signatures in continuous view

Two separate but related issues fixed:

1) width measures with key signatures not rendered correctly if top staff contains either no key signature (eg, a drum staff) or only a generated key signature (eg, generated key signature at start of a system in page view that happens to coincide with a "real" key signature change in one staff only)

2) continuous view not correctly accounting for width of header, resulting in "cramped" rendering of the first measure even for single staves if no key signature