Render from ptcl rather than original scene.
A fairly simple approach, but it adds the translation (not tested yet in scene encoding) and does bounding box culling.
Output of kernel 1 is validated by simple inspection, next step is to wire it up properly.
WIP
Populates the piet-gpu subdir, with an extremely simple renderer. The main program saves the image to a PNG. Contains a few fixes (I was confused about the need for multiple bindings, as opposed to multiple descriptors within a binding).