Fix #17891: Instrument names follow largest stave spatium in part

[miiizen]

Resolves: #17891
Resolves: #19577

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[RomanPudashkin]

This implicitly overrides EngravingItem::spatium(). You should make EngravingItem::spatium() virtual and override it explicitly. This type of solution may not be perfect either, but at least it is more obvious, and you no longer need that cast inside TextFragment::font

[miiizen]

Making EngravingItem::spatium() virtual was my first approach, but after Michele asked me to do a performance test we found that layout took noticeably longer (3-4%). I can change the name of the method to avoid confusion, though we'd keep the cast.

[mike-spa]

Yeah, making spatium() virtual is very elegant codewise, but spatium() is an extremely hot function, and my concern was that resolving the vtable at every call would carry a noticeable runtime cost (which seems to be indeed the case), so I advised against making it virtual. I agree that overriding the method implicitely may cause confusion though, so it's probably better to just call it a different name

[RomanPudashkin]

Yes, you are right. We just should rename InstrumentName::spatium()