Bugfixes [#35263], [#35382] NLA Animated Influence is ignored if strips below
have zero total influence

Previously, when evaluating the NLA stack at a particular point in time, if a
channel hadn't been encountered before, influence values were simply ignored
when accumulating the values contributed by each strip to the overall stack.

This behaviour simplified the handling of the problem of what "baseline" to
blend relative to (i.e. influence basically scales the magnitude of a scalar
around 0, but we may not exactly want a property to get it's value set to 0 as
baseline). However, the problem was that this meant that you'd get popping
artifacts when the a lower strip finally reaches influence=0 but your upper
strips haven't fully reached maximum yet ([#35382]). Another problem was that
you'd end up with less ability to scale the influence of all strips (as in
[#35263]).

So, as a stop-gap fix now, we will allow influence scaling to work on these
strips too. This still doesn't fix some of the other problems regarding
baselines/rest-poses and deterministic behaviour when some channels are only
keyed in one strip which isn't set to extend it's influence... Fixing those
issues is a bit more involved, and would require a bit of refactoring of how we
keep track of accumulation channels.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57333 954f8c5b-7b00-dc11-b283-0030488c597c