Drawing to a texture is done via render passes. Basically you draw by specifying arrays of things you want to draw, but there's an (optional! Ignore if you don't need it!) array of these arrays, which we refer to as a render pass. Each of those specifies a name, which will end up rendering to a texture instead of the screen, and then later drawing (even later passes in the same frame) can draw from that name instead of an image that was loaded from disk.
Right now we don't offer shaders because we're trying to keep this simple, but we may lose that fight eventually. :)
Debugging is extremely basic right now. A lot of puts() debugging right now. This is something we want to improve, though!