Enable use of label() in a graph

[BruceSherwood]

It would be helpful to support the use of a label object on a graph; graph objects (lib/glow/graph.js) already have label attributes that contribute to a "legend" beside the graph, but this is about placing a text-containing label object on the graph to indicate some interesting element of that graph. The way label objects work on canvases is that in lib/glow/canvas.js a transparent 2D canvas is placed in front of the 3D WebGL canvas and the 2D text is displayed in that front canvas.