Cache waveform pixiJS graphics rendered at different zoom levels for better performance

[micbuffa]

Zoom in/out can be slow when the canvas is large and audio buffer length important. Try to cache rendered waveforms in order to speed up rendering when zooming in/out

[micbuffa]

It should be possible to optimize the zoom to be real-time (just redraw what is on the screen, like in video games with horizontal scrolling). In that case, zoom sliders would be great :-)