Output only latest spatial layer if --alllayers 0

Right now, --alllayers 0 will only output operating points that exactly match the largest one in the sequence header. However, in certain cases, the largest one might not be available, and a smaller one should be returned to the user instead. This matches update_frame_buffers() in aomdec to output only the latest frame if --alllayers 0 is specified.

This MR supersedes !892 (closed)