Drop onAnimate frames when dt is too small

[p0nce]

Making a limit of accumulated 20ms before calling onAnimate can save CPU at little visual cost. Could be a counter inside UIElement that is checked before calling onAnimate (eg: 20ms).

• [x] Do we want that UI-wide, or for each single UI Element? => I don't think it makes sense UI-wide
• [x] What's the use case? It can be "no short animation steps" or "fixed animation step" => no short animation steps is kinda more needed
• [x] At the same time, a global time measure on animation CPU cost can be helpful too. => much less so that (say) a flame graph that show widget display cost

Use case 1: Note: high feedback plugins may want to synchronize feedback in the same sync queue, in order to minimize scheduler lock (exiting a tryLock is a heavy operation anyway). And then it is helpful to have onAnimate for children called just after main gui.d onAnimate. Such a UI might need both "do not animate for low dt" and "animated with fixed dt" at the same time, and will batch setDirty calls likewise.

[PolyVector]

Could be useful both UI-wide, and per element (use the greater of the two?). I would still want the ability to attempt 60fps (<16.6ms) for small elements though, so maybe the limit could be optional?

[p0nce]

Could be a UIElement field (another one) that says "don't animate me unless for at least N ms", and then they accumulate an animation "debt". Fixed timestep can be left out I think.

• [ ] Implement that, 0 is default (no minimum)

Note: accumulating double dt means when the debt is paid back, the dt don't add up correctly though.