These are the basics. Still to do are fixing the alpha channel blending, the rect handling,
a texture type to avoid consuming the image over and over again. And more :-)
The idea is that they can be recursively rendered and propagate at least
the transform but also clip in the future.
One thing that's left is to split RenderNode into RenderNode and
RenderNodeMut, so that the rendering itself can be done on a non-mutable
tree reference.
