[vtkusers] Saving image with transparent background from Qt

[Ben Boeckel]

On Tue, Mar 06, 2018 at 17:45:41 +0100, Ignacio Fernández Galván via vtkusers wrote:
> Can anyone get it to work?

ParaView does it this way:

    https://gitlab.kitware.com/paraview/paraview/blob/master/ParaViewCore/ServerManager/Rendering/vtkSMViewProxy.cxx#L474

Basically it screenshots twice, once with a black background and again
with a white background to figure out what is "visible" and subtracts
them.

--Ben