Initial support for prefab preview thumbnails

[vhristov]

This is an early (WIP) request for comment on preliminary (and very limited) support for prefab thumbnails ( #1722 ).

Currently the prefab assets don't include any visual hint that there are any spine skeletons in them.
The reason for this is basically that when saving a asset results in MeshFilter's sharedMesh being invalid. This can be caused by one of the following reasons (depending on the current state of the SkeleonRenderer object):

• Mesh being a scene only object (it cannot be used in for asset preview) and
• Mesh being destroyed after prefab is saved (by disposing the double buffered meshes)
• Mesh is null

The fact that the dynamic mesh is scene only object results in type mismatch mesh (partially handled by #1387 ) which then results in null mesh on the subsequent prefab modifications.

This change introduces an extra asset next to SkeletonData asset, that holds the a mesh in SetupPose which is attached to the sharedMesh when prefab is being saved. This way a thumbnail is generated with the default setup pose for every spine skeleton inside a prefab.

This pull request is more of an initial proof-of-concept and a scream for help

Known things that needs to be completed (and verified):

• Skins support - currently only default skin's mesh is generated (could work in some cases)
• Dynamic mesh save support when editing prefab and attaching it instead of the setup pose mesh
• Fix player build with mesh.hideFlags = HideFlags.DontSaveInBuild
• Split mesh and multiple atlas support (what is this actually?)
• Skeleton Graphics support (is it even possible to have prefab preview for canvas?)
• Check unity compatibility (and possibly hide everything behind a define)
• Reimport prefab inconsistent warning
• What other stuff needs to be supported to prevent random editor failures

While I haven't check a lot of possible scenarios, the simple change presented here seems to work in the most simple use-case. I don't know if it could create any fatal error, but I guess it can have a corner case where an error is logged to the console while saving a prefab. At least on the spine examples I imported nothing broke during the import.

What I mostly want to do as a next step is to enable dynamic mesh save support (I believe this will also fix skin support?)
I imagine this to happen by adding the mesh to the prefab asset (using AssetDatabase.AddObjectToAsset(mesh, prefab) and updating it when changed. I tried to do that by scraping out the double buffer and pointing the mesh to an asset.
What I failed to achieve here is two thing:

• to detect when actually a spine skeleton is edited inside a prefab. I believe Experimental.SceneManagement.PrefabStage.IsPartOfPrefabContents() is a good start, but unfortunately It fails to run while called in Awake or OnEnable and I am uncertain how can to handle the mesh substitution after that.
• to detect prefab creating and substitute the buffer with proper data

Another issue with the above approach I see is how to handle the embedded mesh assets inside the prefab. I haven't come up yet with an idea how to clean up the garbage (for example when a skeleton is deleted from a prefab), how to resolve name collisions (with multiple skeletons inside a prefab), nor how to handle nested prefabs.

Any suggestions on how to proceed further with this are welcome (also any comment on the above checks too)

[vhristov]

After posting the previous message I realized it is possible to reuse the same logic in the asset postprocessor, but instead of attaching a pre-generated asset file, it can be used to generate and store in the prefab a new one. As far as I can tell it solved a few of the issues mentioned above - it supports dynamic mesh and skins.

[vhristov]

As it turned out this pull request breaks player build. For this reason I am closing this pull-request for the moment to prevent anyone cherry-picking it and realizing their builds are broken by it on a later state.