Add method to allow generating family models for existing blocks

patches/net/minecraft/client/data/models/BlockModelGenerators.java.patch

@@ -0,0 +1,19 @@
+--- a/net/minecraft/client/data/models/BlockModelGenerators.java
++++ b/net/minecraft/client/data/models/BlockModelGenerators.java
+@@ -5819,4 +_,16 @@
+             return this;
+         }
+     }
++
++    /**
++     * Neo: create a {@link BlockModelGenerators.BlockFamilyProvider} which re-uses the existing model of the given full
++     * block instead of creating a model and blockstate file for it. Intended for use cases where the full block is
++     * separately generated or otherwise exists such as when a dummy {@link BlockFamily} is used to create additional
++     * variants for existing vanilla block families
++     */
++    public BlockModelGenerators.BlockFamilyProvider familyWithExistingFullBlock(Block fullBlock) {
++        var provider = new BlockModelGenerators.BlockFamilyProvider(TextureMapping.cube(fullBlock));
++        provider.fullBlock = ModelLocationUtils.getModelLocation(fullBlock);
++        return provider;
++    }
+ }

tests/src/main/java/net/neoforged/neoforge/debug/block/BlockTests.java

@@ -10,7 +10,6 @@
 import net.minecraft.client.data.models.BlockModelGenerators;
 import net.minecraft.client.data.models.ItemModelGenerators;
 import net.minecraft.client.data.models.ModelProvider;
-import net.minecraft.client.data.models.model.TexturedModel;
 import net.minecraft.core.BlockPos;
 import net.minecraft.core.Direction;
 import net.minecraft.core.Holder;
@@ -94,8 +93,7 @@ static void woodlessFenceGate(final DynamicTest test, final RegistrationHelper r
             @Override
             protected void registerModels(BlockModelGenerators blockModels, ItemModelGenerators itemModels) {
                 var family = new BlockFamily.Builder(Blocks.IRON_BLOCK).fenceGate(gate.value()).getFamily();
-                var textureModel = TexturedModel.CUBE.get(Blocks.IRON_BLOCK);
-                blockModels.new BlockFamilyProvider(textureModel.getMapping()).generateFor(family);
+                blockModels.familyWithExistingFullBlock(family.getBaseBlock()).generateFor(family);
             }
 
             @Override