Add smarter getter for block slipperiness (#4116)

Fixed MC-119811, Boats rubber banding when dismounted.

src/test/java/net/minecraftforge/debug/SlipperinessTest.java

@@ -0,0 +1,42 @@
+package net.minecraftforge.debug;
+
+import net.minecraft.block.Block;
+import net.minecraft.block.material.Material;
+import net.minecraft.block.state.IBlockState;
+import net.minecraft.entity.Entity;
+import net.minecraft.entity.item.EntityBoat;
+import net.minecraft.item.Item;
+import net.minecraft.item.ItemBlock;
+import net.minecraft.util.math.BlockPos;
+import net.minecraft.world.IBlockAccess;
+import net.minecraftforge.event.RegistryEvent;
+import net.minecraftforge.fml.common.Mod;
+import net.minecraftforge.fml.common.Mod.EventBusSubscriber;
+import net.minecraftforge.fml.common.eventhandler.SubscribeEvent;
+
+@Mod(modid = "slipperiness_test", name = "Slipperiness Test", version = "0.0.0")
+@EventBusSubscriber
+public class SlipperinessTest
+{
+    public static final Block BB_BLOCK = new Block(Material.PACKED_ICE)
+    {
+        @Override
+        public float getSlipperiness(IBlockState state, IBlockAccess world, BlockPos pos, Entity entity)
+        {
+            return entity instanceof EntityBoat ? 2 : super.getSlipperiness(state, world, pos, entity);
+        }
+    }.setUnlocalizedName("boat_blaster").setRegistryName("boat_blaster");
+
+    @SubscribeEvent
+    public static void registerBlocks(RegistryEvent.Register<Block> e)
+    {
+        e.getRegistry().register(BB_BLOCK);
+    }
+
+    @SubscribeEvent
+    public static void registerItems(RegistryEvent.Register<Item> e)
+    {
+        e.getRegistry().register(new ItemBlock(BB_BLOCK).setRegistryName(BB_BLOCK.getRegistryName()));
+    }
+}
+