Suggestion : fix rotation on z axis when the board is upside down

[Gothor]

Hi !

I just discovered your tool, and it looks quite amazing ! So I just wanted to say "Congratulations" first !

So, I noticed that when the board is upside down and you try to turn around the Z axis, then the rotation seems a bit awkward as the board seems to turn in the reverse direction of the mouse.

So here's the little fix I would like to propose : the commit.

Have a nice day !