[Spine] Ability to control alpha of animation tracks

[iampremo]

Ticket #208832

Feature Type: feat_addition_ingame Description: Being able to have more control over how animations are blended together, similar to the SkeletonMecanim component in the unity runtime http://en.esotericsoftware.com/spine-unity Benefit: This would allow to have much more control in code over how animations look in the game and being to able to create brand new animations from using multiple tracks that blend differently.

[SimpCarlWheezer]

In addition to alpha, the ability to choose what happens to the previous animation data when switching to a new animation is essential. This feature is possible in the official runtime of Spine by using the holdPrevious, mixBlend, and shortestRotationproperties. The following links will be useful for more information : https://esotericsoftware.com/spine-api-reference#TrackEntry https://esotericsoftware.com/spine-preview#Tracks Spine Editor Guide