Low priority feature request: add flag to remove XYZ axes from bottom left corner of save_screenshot

[MatthiasJ1]

Personally I feel like saving just the shown objects should be the default behavior. I was surprised to see the additional elements.

[bernhard-42]

Normally, one would have one canvas for the viewer and one canvas for the xyz axes.
However in webGL this means 2 WegGL contexts of a typical maximum of 16 in the browser. Since the viewer component also runs in jupyterlab, where each cell could have a different viewer, I combined cad view and xyz view into one canvas (threejs can define rectangles, i.e view ports, to show the right stuff).

Getting a screenshot in threejs means triggering update to draw the latest into the buffers and then store the canvas buffer as image. Since the xyz axes are part of the same canvas, they will be stored, too.

I think I can make the xyz axes and labels transparent before taking screenshot and then show them again afterwards. Need to check.