_maskContent interferes with pointer events

[macmccann]

I've managed to get my environment all mucked up, so providing a minimally reproducible example is going to be pretty hard. But I'm trying to do various cursor (change cursor, set onMouseDown) events on elements inside the scrollbox, and the final barrier was the this._maskContent graphic that wasn't propagating events down to the PIXI sprites I had added. Just wanted to open this up for discussion, not sure if there's anything immediately actionable.