There are 2 different ways to approach this. First would be a RenderTexture. I actually just put a few examples into the dev branch showing how to use them, so you could start there, but essentially you can paint any Sprite (or Group) to a RenderTexture. In Canvas it creates an "offscreen" canvas, in WebGL it writes direct to the frame buffer, so it's really fast. It's limited in that  you can only draw image data to it though.   I've also added a new object (again only in dev right now) called