replace png() width and height with magnification

[danlipsa]

vtkWindowToImageFilter which is used in png() to get an image from the window supports a parameter int magnification which allows the user to save a larger image than the window size.

Note this does not change the width/height ratio, so probably we won't need to replot anything in changeEvent. This will get rid of a large chunk of tricky code and probably speedup computation when resizing is involved.

It will involve an incompatible change and resizing will be more limited it maybe worth investigating.

@doutriaux1 @aashish24 @sankhesh @williams13 What do you think?