Add a perspective transform option, render gcode views with perspective transform

[kliment]

When the perspective option in settings->options->viewer is enabled, renders the gcode viewer with a perspective transform instead of an isometric transform. Fixes #1239

[volconst]

@kliment , I do not feel competent here, so I trust your judgement :)

[kliment]

@volconst it works okay for me, but I wanted someone else to check it to make sure I haven't missed some obvious breakage that isn't obvious on my setup

[volconst]

I will try to run it in the next 2 hours.

[kliment]

No hurry, it can wait

[volconst]

@kliment , works fine on Ubuntu 20.04

[kliment]

As it's off by default I'll merge it for now, and then we can consider whether we want it on by default

[volconst]

It's nice to see how slicers do it.
I looked at an year old Cura, and they use perspective view. Not sure if it can be switched.
My intuition is that orthogonal is better, because it is more suitable for plate layouting. But maybe people spend more time not layouting, but waiting for the printer to finish the print :)
If so, after print button is pressed it should go to perspective - just kidding :)