in ofxUIScrollbarCanvas, when the contentHeight is less than or equal to the overall init_h of the canvas, then make the scrollbar the entire height of the canvas because in this scenario it does not make sense to scroll. TODO: do not draw the scrollbar at all in this scenario
do not need to offset widgets in ofxUIScrollbarCanvas by the init_y because the parentRect of the widget is the rect that has y top init_y
fixed some height issues in ofxUIScrollbarCanvas