Extract a new interface, ScatterPlotWebGLVisualizer, that contained the

remaining protected methods from ScatterWebGL. Rename Scatter to ScatterPlot.
ScatterWebGL is now ScatterPlotWebGLContainer, is concrete, and dispatches its
previously abstract methods to an array of ScatterPlotWebGLVisualizer objects. Rename ScatterWebGLPointsCanvasLabels.ts to ScatterPlotWebGLVisualizerPointsLabels.ts.

Move getProjectedPointFromIndex and vector3DToScreenCoords to util.
Change: 133615506