true if the curve was calculated and can be drawn, false otherwise (e.g. missing notes, or the two staves are not in the same MusicSystem - a cross-staff plus cross-system slur is not supported).
StaticCompareCompares the timespan of two Graphical Slurs
Calculates the bezier curve for a slur that crosses between two staves (e.g. left hand to right hand), where the start and end notes lie on different stafflines that are stacked vertically within the same MusicSystem. Unlike [[calculateCurve]], this runs at draw time, because it needs the final vertical positions of both stafflines, which aren't fixed until the system Y-layout (after calculateSlurs()).
The resulting bezier points are stored relative to the start note's staffline, so the regular drawSlur() (which adds that staffline's absolute position) renders them at the correct absolute location.