Fix seeking Animation immediately after playback for Discrete track

[TokageItLab]

• Fixes Playing backwards discrete animation will begin with the first keyframe #92840
• Follow up Fix Deterministic blending with Dominant/Recessive doesn't have initial value even if there is no Discrete track #92126
• Supersedes/Closes Fix playing backwards discrete animation will begin with the first keyframe #92846 (comment)

Reorganized #92126. Also, I believe the #92840 problem is mainly related to the editor. The seek(0) by stop() was processed incorrectly and the negative value for reverse playback was not propagated to the AnimationMixer on the first frame immediately after playback.

[KoBeWi]

It does not fix #92840
If you run the MRP the animation is still broken.

[TokageItLab]

Oh, it may not be essentially a Discrete issue. I recall a problem being reported somewhere earlier where the beginning of the animation was processed when playing backwards from a position beyond the length of the animation. At least for the Discrete playback issue, this PR will fix that.

[TokageItLab]

@KoBeWi Okay, I see the problem, #92840 is not only in backward, but also in normal playback, where the RESET track is being played instead of the first frame if there is no key there. This is the intended behavior for deterministic, but if deterministic is disabled, the intended behavior should be to do nothing. I added a fix for that.

When Deterministic is enabled, RESET may not occur depending on the way the backward playback, but this is a problem that depends on the behavior of the Discrete key by seeking and the deletion of the cache by stopping, well, the complex combination of the Discrete key and Deterministic is considered to be a rare case so I assume it is okay for now.

[KoBeWi]

Ok now it's fixed. I forgot to mention that my original issue occurred without RESET animation, but it doesn't seem to make difference.

[TokageItLab]

I also added a fix for the regression of the backwards seek (mainly for editor), which seems to have been occurred in dev5.

I feel that the other track types should be fixed as well, but if we do that, we need to handle all the events between seeks as mentioned in #92805 (comment). Although for Discrete, this is not necessary since it will be overriding only one property.

[akien-mga]

Thanks!