Fix mushrooms blocks on 1.12.2 and below (#351)

common/src/main/java/nl/matsv/viabackwards/protocol/protocol1_12_2to1_13/data/BackwardsMappings.java

@@ -106,6 +106,25 @@ public class BackwardsMappings extends nl.matsv.viabackwards.api.data.BackwardsM
         }
     }
 
+    @Override
+    public int getNewBlockStateId(int id) {
+        int mappedId = super.getNewBlockStateId(id);
+
+        // https://github.com/ViaVersion/ViaBackwards/issues/290
+        switch (mappedId) {
+            case 1595:
+            case 1596:
+            case 1597:
+                return 1584; // brown mushroom block
+            case 1611:
+            case 1612:
+            case 1613:
+                return 1600; // red mushroom block
+            default:
+                return mappedId;
+        }
+    }
+
     @Override
     protected int checkValidity(int id, int mappedId, String type) {
         // Don't warn for missing ids here