Idea:
1. in its mutable recording state, keep a table of drawables on the side, and store an index in the record list.
2. In "immediate-mode" draw, just call the clients drawable directly (need access to our private list to turn the stored index into a proc)
3. when we "snap", we replace the list of drawables with a list of (sub) pictures, and then during playback of the snapped picture, we invoke a private drawable which just calls "drawPicture" on the index'd subpicture.
Review URL: https://codereview.chromium.org/727363003
6be2aa9a25