Blender Git Loki

NLA: Extract nlasnapshot_blend()

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

**Motivations**:
* {T83615} 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} 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 User-side Functional changes

Reviewed By: sybren, #animation_rigging

Differential Revision: https://developer.blender.org/D10220