fix(face-landmarks): dispose tensors to avoid memory leaks

Also send only expressions with score grater than 50%.

react/features/face-landmarks/faceLandmarksWorker.ts

@@ -99,15 +99,22 @@ const detectFaceBox = async ({ detections, threshold }: Detection) => {
     return faceBox;
 };
 
-const detectFaceExpression = async ({ detections }: Detection) => 
-    detections[0]?.emotion && FACE_EXPRESSIONS_NAMING_MAPPING[detections[0]?.emotion[0].emotion];
+const detectFaceExpression = async ({ detections }: Detection) => {
+    if (!detections[0]?.emotion || detections[0]?.emotion[0].score < 0.5) {
+        return;
+    }
+
+    return FACE_EXPRESSIONS_NAMING_MAPPING[detections[0]?.emotion[0].emotion];
+}
+
 
 const detect = async ({ image, threshold } : DetectInput) => {
     let detections;
     let faceExpression;
     let faceBox;
 
     detectionInProgress = true;
+    human.tf.engine().startScope();
 
     const imageTensor = human.tf.browser.fromPixels(image);
 
@@ -131,6 +138,8 @@ const detect = async ({ image, threshold } : DetectInput) => {
         });
     }
 
+    human.tf.engine().endScope()
+
     if (faceBox || faceExpression) {
         self.postMessage({ 
             faceBox,