Blender Git Loki

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.