Do frame interpolation only once at the end of all <extend>(Fixes #994)

[willhsmit]

• Add code to the I2V flow to skip frame interpolation if there are additional prompts (T2V already has this check).
• Add code to the extend flow to do frame interpolation after the last extend. There are separate T2V and I2V interpolation method and multiplier, so check whether VideoModel is set to pick the appropriate one.

Confirmed that for all of: T2V with no extends and with 1-2 extends I2V with no extends and with 1-2 extends

there is exactly one RIFE VFI node before the last SwarmSaveAnimationWS with the right multiplier, and the unmultiplied fps on any prior save and the multiplied fps on the last save.