Add back missing Scratch3VideoSensingBlocks.INTERVAL

The INTERVAL time defines how often to analyze a frame. Without the value the
test whether a frame should be analyzed will always be false.

src/extensions/scratch3_video_sensing/index.js

@@ -92,6 +92,15 @@ class Scratch3VideoSensingBlocks {
         this._loop();
     }
 
+    /**
+     * After analyzing a frame the amount of milliseconds until another frame
+     * is analyzed.
+     * @type {number}
+     */
+    static get INTERVAL () {
+        return 33;
+    }
+
     /**
      * Dimensions the video stream is analyzed at after its rendered to the
      * sample canvas.