There was a thread in the Godot forums a while back on porting Godot to the Pi, but interest seems to have died out. It's a real shame too, because Godot is shaping up to be the
best 2D engine out there, and it's open-source to boot! It would be the ideal choice for the Pi. Maybe interest will be rekindled after Eric Anholt's open-source driver is finally implemented. You could also make a post there as well, here's the link to it: http://www.godotengine.org/topics/7369?page=1
The only other engine I know of that officially supports the Pi is the Urho 3D Engine.
Which, by the way, is the first
result when googling "Raspberry Pi game engine".
EDIT: It seems x1212 actually returned not too long ago to continue the porting efforts of Godot, as seen here: http://www.godotengine.org/topics/12221
If anyone is interested in using Godot on a Pi, I would recommend you head over there and offer some encouragement, or help if possible.