[Experimental Add-on] True Onion Skinning (Patreon)

Hey everyone!! This is our first ever Experimental Release for some tools that we've developed but haven't had time to test extensively. I want to thank everyone who's been part of the experimental tier, and I hope this is useful to you in some way! 

Over the years, Blender has *never* had a good Onion Skinning tool for 3D animators. I know one is being worked on but may not be finished for years as they overhaul the animation workflow as a whole.

However, we stumbled upon an add-on by Andrew Combs called Animation Extras that does do quite a good job. But when testing it out, I found that it lacked a lot of key features. So we edited it for usability and made it one of simplest, but most functional Onion Skinning add-ons out there (in my opinion).

We hope to release this add-on for free in the future, but we never got to stress test it. And I wanted you guys to have a chance to try it out! Download it here or in the attachments down below.

Here's a quick tutorial:

- Select the object(s) you want to onion skin.
- Press Set Selected to Onions. Note: there must be keyframes on the object or the armature it's parented to for it to respond
- Then press Draw
- Whenever a change in the animation is made, press Update All
- Tinker with the settings as you like
- Press Stop Drawing anytime to stop
- Add/Clear Selected let you add or remove objects from the Onion Skinning "set" of objects. You can also press Clear All to remove all objects from Onion Skinning.

WARNINGS:
- Turning it on for heavy meshes will likely lag your scene. It's recommended to only use this for low poly proxy versions of your characters.
- Overall, it can slow down your scene if it's too heavy, as it's technically drawing your object multiple times. This is one of the major drawbacks of all 3D onion skinning add-ons--it's processor intensive.

Here's Andrew's original add-on for comparison https://andrewcombsdesigns.gumroad.com/l/animextras 

- Doesn't apply Onion Skin to multiple selections of objects
- Doesn't have a way to add and remove objects from the Onion Skinning "set" of objects
- Update button applies to all objects in the Onion Skinning "set"
- We also made some other small quality of life changes. Overall, though, the fundamental mechanics are his.