scene: First step towards what I think is a good ViewBox
scene: first version working with new viewbox/drawing mechanism.

Only draw viewbox via vierport yet.
scene: viewbox now works via a transform
gloo: fixes to framebuffer
(is this thing even tested?)
scene: viewbox now works via FBO!

Although in a sort of hacky way.
We now have all three ways implemented.
scene: cleaning up and doc
scene: bit of refactoring and cleaning

the PaintEvent is now pretty nice IMO. The DrawingSystem very nimble, which
I also like.