improve sprite init from score

[matthewdean]

Changes:

• load behaviors correctly from score
• load correct sprite init data for the given frame
• in the frame loop, advance the frame regardless of whether it has been skipped (please double-check this is desired)
• support go(frame_label) instead of just go(frame_num)
• switch from setting sprite cast_lib and cast_member properties to calling sprite_set_prop

[matthewdean]

Now also deserializing film loops, which use the same memory format as score chunks.

In the cast member specific data section there are many fields which are present for film loops but which appear to be only relevant to other cast member types such as movies. I may see about generalizing the parsing code so it works for all.

[igorlira]

I had to merge this manually due to conflicts so I'm closing the PR, but I've checked that all of its commits have been merged into main.