In this first step, we just "blindly" move everything under src/driver
to a new library libbackend.a. And all headers are moved under
private/backend.
Note that "driver" is renamed "backend", but namespaces are unchanged for now.
Later we'll have to untangle the actual private headers from public
ones and ideally not have any private headers.
* Add material_testbed application
This application can be used to automatically vary material parameters over multiple frames and capture each frame as a PNG file. This makes it easier to test/compare/etc. It's also useful to generate documentation examples.
* Rename material_testbed to frame_generator
