Camera attributes: document, exemplify, test

* Document the specific names that should be used for common camera parameters, when retrieving with getattribute().

* Add the ability to retrieve these in testshade & testrender, and add a testsuite test to exercise it. The implementation in testshade/testrender also serves an example of how to do it in a real renderer.

* Various fix-ups make certain parts of testshade and testrenderer identical again, in places where they had needlessly diverged.