[3.x] Add options for sorting transparent objects

[QbieShay]

This commit adds two properties to visual instance:

• sorting_use_aabb_center default to true, use the aabb center for sorting. if set to false, it uses the origin (pivot) of the object insted.
• sorting_offset a float value that will change the depth that is assigned to the object in the transparent queue.

This is especially useful for VFX where there are a lot of particle systems on top of each other, giving artists the full control over the ordering of particles within the same vfx, since godot has no concept of a vfx. (this can't be done with render priority, see: godotengine/godot-proposals#4099 )

I'd love to port this to 4.x but I will need someone's help to figure out where i should touch the renderer.

Video of the new behaviour https/giant.gfycat.com/MetallicTenderFlounder.mp4

This work was kindly sponsored by Pahdo Labs

[akien-mga]

@reduz approved the feature in PR meeting but wants it checked in depth by @godotengine/rendering folks.

[akien-mga]

You should also add documentation for the new properties: https://docs.godotengine.org/en/latest/community/contributing/updating_the_class_reference.html

[clayjohn]

Looks good to me subject to Akien's comments and my one comment and adding docs

[clayjohn]

Great work! This is going to be super useful for many people :)

[YuriSizov]

More of a nitpick in 3.x, but it's preferred to define the default values in the header instead of the constructor. This should definitely be the case in the master version, if there is one.

[akien-mga]

Yeah for 3.x I let this go through as this is actually more consistent, since we didn't port most of the codebase.

[YuriSizov]

Left a few notes, most are nitpicks, but the docs have some confusing wording worth correcting before merging.

[YuriSizov]

Thanks, everyone! Let's make the master version though :)

[timothyqiu]

Cherry-picked for 3.5.2