53b03eff96
If you have two overlapping NLA strips it automatically blends between them.
However it was also blending between the first strip and the rest pose
(e.g. 0,0,0 for locations and 1,1,1 for scale).
This is ok if the blend mode is Add or Multiply since then you are adding onto
the rest pose, but for Replace you want to be able to mix between two poses
and not take the rest pose into account at all.
What this does is that to not do any blending for the first strip if it has
blend mode Replace. If someone wants the old behavior (from bugfix commit
27d792fa9c) they can still make an action with the rest pose and use that
as the first strip.