Fix: wrong position and scale of context items when resizing ImageViewer (vtk6)

[mdkus]

#1049 ...I think the only way to get the same resize behaviour than with 'vtkImageSlice', is to derive a new class of the context items context actor, which is drawing them, und to override its drawing function. After some testing I can say that this emulation function ('RenderOverlay') leads to precise results:

Before patch:

After patch:

[mdkus]

@taketwo Sergey?

[taketwo]

Maybe we could factor out this version-dependent code into a separate class? For example, it could be PCLContextActor and go into "visualization/vtk" folder. For the old vtk version it would be simply a typedef, and for the new one your proposed class. Then we will avoid adding new #ifdefs in the viewer code. I know it is stuffed with conditional compilations already, but why adding more? What do you think?

[mdkus]

Yes, I agree. I'll prepare a new pull request.

[taketwo]

Thanks!