fix #93006: layout shift due to overestimating size of courtesy keysig

[MarcSabatella]

Last year we (mostly @mgavioli ) fixed a number of issues that were causing inconsistencies in layout, with a typical symptom being that each time you pressed Ctrl+A, one measure would move back and forth from one system to the next, with a correspond ripple effect on layout from that point on. These issues all had to do with how we accounted for the width of courtesy elements - clefs, key and time signatures.

In the process of fixing these, I introduced some code that almost but not quite worked:

25e43f2

It works well if there is only one staff, but the more staves, the more the code overestimates the width of courtesy key signatures, because I keep adding leftMargin to the accumulated max width on every iteration of the loop rather than only adding it to the current width. So the more staves a piece has, the more we overestimate the width of courtesy elements, leading us to think a measure will fit that won't or vice versa.