Hacked iOS 9's Picture-in-Picture mode (codenamed Pegasus) to play Doom over top all other apps (including SpringBoard) without any jailbreak or process injection :D
Notably, Bell says that "This can be used (and abused!) with public API to place any kind of content in a PIP window :D"
The Picture in Picture feature is designed to let users continue watching a video or use FaceTime while performing other tasks on the iPad, such as checking their email or browsing the web.
Take a look at Bell's video below...