exporter: Refactor force_root_clip_play to prevent force play from looping.

[AeroXuk]

Split from #20896.

• Improve the --force-play feature to avoid restarting the timeline if it's already on the last frame.

[kjarosh]

What's the purpose of it? Will that be even necessary after #20929 ?

[AeroXuk]

What's the purpose of it? Will that be even necessary after #20929 ?

#20929 is adding a way to exit the export loop early if we reach the end.

Using the following senario, if you request it to export 200 frames of a 100 frame animation:

• Without using --force-play, you will have frames 1 to 100, then 100 copies of the last frame.
• Currently with --force-play, you will have frames 1 to 100 twice as it loops back to the begining.

The current looping behaviour with --force-play was not intended, the feature was intended for bypassing "click to play" or "click to continue" prompts. Adding this bugfix, would make the output match what happens if we aren't using the --force-play, once we reach the end we just keep outputting the last frame.

This then makes #20929 a bonus feature to also allow the exporter to stop after the last frame was exported, regardless of how many frames are requested.