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NLA: Extract nlasnapshot_blend_get_inverted_upper_snapshot()

Extracts //nlasnapshot_blend_get_inverted_upper_snapshot()// from //BKE_animsys_nla_remap_keyframe_values()//

- This introduces a new struct member: `NlaEvalChannelSnapshot->invertible` which marks values which have successfully been inverted and can be further used for solving.
- //nlasnapshot_mark_all_invertible()// is unused in this patch but seemed generally useful when using snapshots for remapping. {D8867} would be able to make use of it.
- //nlasnapshot_blend_get_inverted_upper_snapshot()// has a variant //nlasnapshot_blend()// from {D10220}, but this patch doesn't depend on it at all. A third variant will later be added //nlasnapshot_blend_get_inverted_lower_snapshot()//. Altogether, these three functions allow solving for any of (lower_snapshot, upper_snapshot, blended_snapshot) given the other two.

No user functional changes

Differential Revision: https://developer.blender.org/D10222