Animation mode: stretch

[mrtux]

I created an animation that would represent a process in an assembly-like machine, which ideally takes as much time as the assembly process for one item (which varies between recipes).

Instead of keeping a global list of assembly machines and updating the animation speed on every tick, it would be nice to have the animation mode "stretch" (or something along this line) that adapts the animation speed so that the current frame matches the assembly progress.

(I believe this is relatively easy, either by setting the animation speed or by proportionaly setting the current frame depending on the progress.)