performLayout method
Implementation
@override
void performLayout(ChantContext ctxt) {
super.performLayout(ctxt);
build(ctxt)
..virgaAt(notes[0])
..advanceBy(ctxt.intraNeumeSpacing)
..virgaAt(notes[1])
..advanceBy(ctxt.intraNeumeSpacing)
..virgaAt(notes[2]);
finishLayout(ctxt);
}