Add pointer conversion operator to Resource.

Unlike reference conversion this doesn't fire assertion when resource is
not loaded.

src/Resource.h

@@ -177,6 +177,16 @@ class Resource {
             return data;
         }
 
+        /**
+         * @brief Pointer to resource data
+         *
+         * Returns `nullptr` if the resource is not loaded.
+         */
+        operator U*() {
+            acquire();
+            return static_cast<U*>(data);
+        }
+
         /**
          * @brief Reference to resource data
          *