​1.3 Update - PaperPolygon2D Property Update Overhaul + Misc. Tweaks

Changes:

After thinking about it for a good while, I've finally decided to bite the bullet and completely remove per-frame checking on PaperPolygon2D instances. I've decided that it's best for the sake of optimization, and that's at least part of the selling point behind this addon to begin with. Alternate methods have been implemented so that this does not limit functionality. If you've had any changes to the PaperPolygon2Ds be script-based, then this is a breaking change in that regard, and you should probably look at the changes.

PaperPolygon2D Changes:

• New "update" and "set + update" functions. If you've manually set any PaperPolygon2D's Polygon2D values in a script, you may want to update that code to use the new "set + update" functions.
• Property set functions for Polygon2D inspector values are changed to call the aforementioned "set + update" functions. This should allow in-editor and keyframed changes to still be properly set.
• Live z-index updates, with the 3D z-indices also now being recalculated whenever the Polygon Group's child order changes.
• In case if there's something amiss, there's now a "Recalculate Z-Indices" button that calls `skel.mesh_manager.defer_z_index_update()` to manually recalculate z-indices. Ideally though, you should never need to press this.
• `z_as_relative` is now accounted for in z-index calculation.
• Moving the PaperPolygon2D and editing its data arrays in the inspector also now reconstruct its 3D mesh equivalent. Sadly, this does not apply to editing through the actual Polygon2D editor due to limitations.
  • Side Note: This feature mainly has been added just to slightly increase editor functionality and to support niche use cases. Repeatedly reconstructing 3D meshes for the sake of animation is hardly efficient, so I'd highly recommend having vertex distortions instead be done through the actual skeletal system, both for the sake of performance and practicality.
• Added "Reconstruct 3D Mesh" button that reconstructs the PaperPolygon2D's 3D mesh equivalent.
• PaperPolygon2Ds now store references to their equivalent 3D mesh instances and 3D z-indices instead of being bound to two dictionaries within the PaperSkeleton node. This also involved a lot of refactoring to remove references to those two dictionaries. 
• Renaming PaperPolygon2Ds while PaperSkeleton is active no longer breaks functionality with internal shader dictionaries.

Other changes:

• A lot of overdue refactorings in PaperSkeletonMeshManager, with the help of some of the new update functions in PaperPolygon2D.
• Corrected some non-static typing, particularly in PaperSkeletonShaderPreset.
• Minor refactoring elsewhere.
  
• PaperBoneAttachment3D changed to account for when `flip_model` is enabled so that the scale is no longer inverted, but maintains proper orientation. This way, attached meshes will no longer be weirdly flipped.
• Fixed a bone transform modifier bug that made it so that not all of the modifiers would always update due to improper index references.
• Rolled back material caching. Somehow completely neglected the fact that this would lead to more than one instance of the same PaperSkeleton rig sharing the same material, which is bad for texture changes and other things. This is okay though because, for some reason, itch.io tells me that no one downloaded the 1.2 version. This is somehow both relieving and discouraging. If you're reading this, know that the new features are good, and that you take advantage of them! Except when they break, but just tell me when they do, and I'll fix them when I can.