Add handling for vace_context in context windows

[drozbay]

Simply slices up vace_context by window to allow the usage of VACE conditioning with Context Windows. Since the reference image is not separated from the rest of the VACE context, there is no way to know if it includes a reference image. When the retain_index_list parameter set to 0 the reference frame will be applied properly to each context window. Consider exposing this option so that users can use VACE with context windows and reference frames.

[drozbay]

Example using 49 frame context with no reference:

https://github.com/user-attachments/assets/3b7ca938-651a-46f7-8d37-5027693db013

Example WF: droz_VacePose_native_basic.json Input pose video