This adds a new RNA method Action.flip_with_pose(ob, frame)
to flip the action channels that control a pose.
The rest-pose it's self is used to properly flip the bones transformation.

This is useful as a way to flip actions used in pose libraries,
so the same action need not be included for each side.

API calls to cache F-curve look-ups have also been added,
supporting a single hash lookup to access all channels controlling an RNA path.

Motivation

This patch adds functionality to flip an entire action on the X axis using a symmetrical rig.

This was written so the asset manager can apply poses flipped, see T86159,

tested with spring.blend.

Alternative Solutions

It's possible to calculate this on the pose directly (see D10766), however operating on the action data makes this more useful as an API function, since the data can be flipped before it's applied to the pose.
Instead of flipping on the pose, then writing back to the action.

Limitations

There is some information not easily available at the time of flipping, for example - it's possible the flipped data-path doesn't exist in all cases, although the ideal behavior isn't obvious when the RNA path only resolves on one side of the pose.

• Currently the API function flips all channels, we could add an argument so it only operates on some of the channels.

Open Topics

• When some actions transform channels don't exist, they could be created (for example - if only X & Y rotation are set, it may be necessary to create a Z F-Curve to properly flip the rotation).
• The key-framing API is currently part of the editors (it would be a bad-level call from the BKE), we could expose some keyframing functionality to BKE, or move this functionality to editors. For now there is a simple keyframe adjusting function.
• Currently only the armature rest-matrices is used, not the pose. This API could take an armature instead of an object, although we might want to use pose content in the future, it doesn't seem like a big issue either way.
• This could eventually be exposed as an operator.
• Currently this re-orders channels which could have other frames keyed. A more comprehensive approach could be to operate on all keyed frames for an action, so an action that includes multiple poses will have them all properly flipped.

Notes

• The F-Curve lookup cache can be committed separately.

• This is an example script for testing with the pose object set active:

import bpy
from bpy import context
ob = context.object
action = bpy.data.actions['07_040_A.spring']
action.flip_with_pose(ob, context.scene.frame_current)