Fix MusicXML beams with notes of different types
Fix MusicXML beams with notes of different types

cute/vexflow-musicxml,lasconic/vexflow-musicxml,clecap/vexflow-musicxml,lasconic/vexflow-musicxml,clecap/vexflow-musicxml,clecap/vexflow-musicxml,cute/vexflow-musicxml,ringw/vexflow,clecap/vexflow-musicxml,cute/vexflow-musicxml,cute/vexflow-musicxml,lasconic/vexflow-musicxml