We are currently working on a application with similar features called Leonardo although Leonardo is streamlined for digital painting, not photo editing.

Leonardo is still at least 6 months away from being released but all the features like "unlimited canvas", "instant painting" and handling of "huge files" are already in place.

Main website: http://www.getleonardo.com (not much there yet)

We also just started a Vimeo channel were we "blog" about the progress of Leonardo: https://vimeo.com/channels/sol/

Unfortunately there is no video right now showing the "unlimited canvas" and "instant painting", but I can make one within the next couple of hours... :)