[1.21.3] Add event for registering atlases for use with Material

[XFactHD]

This PR adds an event to allow registering texture atlases for inclusion in the AtlasSet held by the ModelManager such that the atlas can be used with Material and other systems that use Minecraft#getTextureAtlas() or ModelManager#getAtlas() to access it.
This is useful for things like using an atlas for an entity-style model like vanilla does for chests, beds, signs and shulkers.

This PR is the 1.21.3 port of #1581.

[neoforged-pr-publishing]

• Publish PR to GitHub Packages

Last commit published: 2290500b81d2c4bd6af49159abc526b1cad357b0.

PR Publishing

The artifacts published by this PR:

• 📦 net.neoforged:testframework:21.3.2-beta-pr-1621-material_atlas_21.3
• 📦 net.neoforged:neoforge:21.3.2-beta-pr-1621-material_atlas_21.3

Repository Declaration

In order to use the artifacts published by the PR, add the following repository to your buildscript:

repositories {
    maven {
        name 'Maven for PR #1621' // https://github.com/neoforged/NeoForge/pull/1621
        url 'https://prmaven.neoforged.net/NeoForge/pr1621'
        content {
            includeModule('net.neoforged', 'testframework')
            includeModule('net.neoforged', 'neoforge')
        }
    }
}

MDK installation

In order to setup a MDK using the latest PR version, run the following commands in a terminal.
The script works on both *nix and Windows as long as you have the JDK bin folder on the path.
The script will clone the MDK in a folder named NeoForge-pr1621.
On Powershell you will need to remove the -L flag from the curl invocation.

mkdir NeoForge-pr1621
cd NeoForge-pr1621
curl -L https://prmaven.neoforged.net/NeoForge/pr1621/net/neoforged/neoforge/21.3.2-beta-pr-1621-material_atlas_21.3/mdk-pr1621.zip -o mdk.zip
jar xf mdk.zip
rm mdk.zip || del mdk.zip

To test a production environment, you can download the installer from here.

[neoforged-releases]

🚀 This PR has been released as NeoForge version 21.3.14-beta.