frameloop never missing tail callback

[RenaudRohlinger]

In a frameloop='never' configuration the tail callback is missing post-render.
Since the render is getting stopped after calling advance() I believe this callback should be called in order to be consistent with the two other modes always and demand.

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[drcmda]

im not sure, tail usually indicates end of a series of frames. if im not mistaken it isn't called in frameloop=always because there's no "end". imo if frames are called manually via advance, tail should probably also be called manually?

[CodyJasonBennett]

We should expose a way to flush effects here, but advance is only concerned with a single frame.

invalidate should otherwise handle this if I'm not mistaken. Are you calling this in a loop?

[drcmda]

IMO invalidate is also no indication for tail. Tail is when rendering is demand and you rotate controls, once it stops calling invalidate tail is called. Invalidate basically just counts a ticker up, even if the ticker was 0 it’s no Guarantee that another invalidate will count it up in the same frame. that ticker is decreased in the render function, which is called after all invalidates, if it reaches 0: tail. Tail can’t be reached when rendering is always. Again, I think tail must just be callable manually, that should do.

[CodyJasonBennett]

Continuing in #2481.