Refactor nlastrip_evaluate_actionclip() and nlaeval_blend_value() into nlasnapshot_blend(), nlastrip_evaluate_actionclip(), and nlasnapshot_from_action().

Motivations:

• T83615: NLA: Strip Blend-space Transforms to Easily Align Actions Requires reading all pose bone fcurves before being able to apply pre-blend transforms. The function nlasnapshot_from_action() achieves this. This effectively removed the need to specially handle Quaternion blend queuing so that code has been removed.
• D8296: Feature: NLA: Evaluate Whole NLA Stack in Tweak Mode Adds support for keyframe remapping through an upper stack of strips. Instead of introducing a variant of the form: nlastrip_evaluate_actionclip_inverted_get_lower(), nlastrip_evaluate_actionclip() will later be extended to take an evaluation_mode as input to avoid duplicating the recursion functions related to nlastrip_evaluate().
• nlasnapshot_blend() will eventually have variants of nlasnapshot_blend_get_inverted_lower_snapshot() and nlasnapshot_blend_get_inverted_upper_snapshot() which are all independent of NlaStrips and NlaTracks, further simplifying the blending implementation. Ideally, nlastrip_evaluate() would get renamed to nlasnapshot_blend_strip() but that'll be a later patch to avoid unnecessary patches slowing the review of more important patches.

No Functional changes