Add PauseAtLastInterrupt input and scene_resume_use_last_speaking cvar

[Blixibon]

Cvar for scene resumption code to use the last speaking actor, rather than always the first; also includes input to pause at last interrupt point, a closely related feature.

This PR adds a new input called PauseAtLastInterrupt which pauses the scene and then rewinds it to the last interrupt point. It also adds a new cvar called scene_resume_use_last_speaking which plays the resume scene with the last speaking actor as !Target1, instead of the first actor in the scene. This PR also fixes resume scenes not using any actor at all in some cases.

These changes are pretty much only relevant to scenes that use the "resume scene" keyvalue to play a resume scene or concept after the scene is paused.

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