scene: remove parent_transform
scene: refactored to use continuous scenegraph with SubScene

Code is a mess, but it took me a while to get here,
so I am "saving" now.
scene: flip y-axis in pixelCamera. Add support for viewport.

Also some cleaning
scene: wip

Working on FBO, but am stuck, now I start to think that the
`reversed` in full_transform is wrong ...
scene: ChainTransform(a,b) == a*b plus many tests
scene: implemented fbo and viewport clipping methods.

They can also be combined.
scene: some cleaning and remove ViewBox.camera

Use ViewBox.scene.camera instead
scene: rename SceneCanvas.root to SceneCanvas.scene
scene: fix that visual examples work again