By my understanding:
A rendering engine simply handles, well, rendering: dealing with low-level graphics APIs, handling shaders, materials and so on, etc. Some may do a little more–event-handling, for example–but the focus remains on rendering.
A game engine provides parts useful for creating a game. This may (and I think very probably will) include rendering, but also includes such things as file-handling, animations (as in the Panda “Actor” class), AI, sound, physics, and so on.
In some cases this may involve aggregating other engines–look for example at Panda’s integration of Bullet and OpenAL.