examples: Add a simple scene graph example
examples: Add a simple scene graph example

3 spinning cubes, who needs anything else? Actually, as I write this
commit message, showing the parent <-> child relationship of transforms
in the example would be probably best. Oh well, next time!

Signed-off-by: Damien Lespiau <64bd3cb94f359c1a3ce68dae5e26b40578526277@gmail.com>

dlespiau/dax,dlespiau/dax