Replay graphics for graphical programs

This is a pretty oft-requested feature and would be a majestic increase of rr's killing power. We've brainstormed a few ways to do this. The most promising way is probably to emulate the X command stream, though of course that's a tremendous amount of work. Perhaps someone has already done that as part of an X debugger? Another way is to let client programs "opt in" to graphics replay by writing presented buffers back to rr somehow. This would be dirt simple but has the disadvantage of slowing down recording and potentially ballooning the trace saved-data.

The issue names no files, tests, or entry points. Start by researching how X command streams and existing X debuggers could support replay, then compare that approach with client programs writing presented buffers back to rr. Done would require choosing and implementing a defined graphics-replay design, including its recording and trace-size tradeoffs.