add drop-item model, new block functions

res/content/base/rigs/drop-item.json

@@ -0,0 +1,5 @@
+{
+    "root": {
+        "model": "drop-item"
+    }
+}

res/content/base/rigs/drop.json

@@ -1,5 +1,5 @@
 {
     "root": {
-        "model": "cube"
+        "model": "drop-block"
     }
 }