Discrete value tracks don't update every frame (only when a new key is
reached). So we can't use the actual property value as an accumulator:
it will end up being zero most of the time.
The _process_node function (which recurses through the blend tree
generating blend values and the active animation list) had an argument
named `switched` which would loop an animation back to the beginning if
it had reached the end (regardless of whether or not it was supposed to
be a looping animation).
This argument was only used in four places: two of them were overridden
by a seek-to-zero, and I believe the other two are bugs.
In OneShot, it was used to reset the oneshot animation to the beginning
when fired. But this would fail if the oneshot node was fired before it
had completed its previous run. While this *could* be a valid way for
oneshot to work (firing does nothing if it's already running), the code
currently resets the fade-in, so I believe that it is intended to reset.
I replaced this usage with seek-to-0.
In Transition, it was used on the previous (fading out) animation when
seeking the Transition node, which I believe is incorrect: why would you
want to loop a non-looping animation instead of simply fading out from
the end? Also it will never happen unless you seek the Transition node
twice during one cross-fade.
The other two uses are in Transition and _process_animation, where it is
used along with a seek-to-zero which overrides it.
When AnimationTreePlayer switches to new animation it never
seeks it to 0 which leads to problems with non-looping animations being
played just once.
This patch is direct approach fixing this problem.
It handles most common cases of occurance.
Closes#2199
