fixed artifacts at frame or crop window boundaries.

problem revealed by commit 094833ddd95b7afbf413819c8432717e89c76f44 ("the crop window is now a property of the frame"), since this commit switches to an always-on crop window, possibly the size of the frame.
removed render layers from test scenes.
updated test suite reference images (default pixel filter is now Gaussian instead of Mitchell-Netravali).