Add unit test for BKE_fcurve_keyframe_move_value_with_handles

Update comments and variable naming

No functional changes.

Fix comment to match current implementation

No functional changes.

Implement Euler filter by using matrices instead of quaternions

The quaternion-to-euler code uses matrices, and that is where the magic
happens. So, instead of Euler ? quaternion ? matrix ? Euler, the code
now does Euler ? matrix ? Euler.

Add unit test

Fix compiler warning

Add check on keyframe X-coordinates

Better reporting after the filter is done, to prevent scary error messages

Reintroduce single-channel filter for completeness.

Improve error messages & levels

Merge remote-tracking branch 'origin/master' into temp-T52744-euler-filter

Cleanup: Animation, fix uninitialised variable in Euler filter code

No functional changes.

New Euler filter implementation

This new discontinuity filter performs actions on the entire Euler
rotation, rather than on the individual X/Y/Z channels only. This should
make it fix a wider range of discontinuities, for example those in .

Manifest Task: T52744

Cleanup: Animation, refactor Euler filter

Split up the Euler filter function into two more functions. The Euler
filter operator works in two stages (find channels that define the X/Y/Z
Euler rotations, and perform filtering on those channels), and each
stage now has its own function. This makes it clearer which data are
used in which part of the code, and makes future improvements easier.

No functional changes.