Normalize cross product result for up vector

At minima, normalizing the up vector obtained from cross product resolves issue CesiumGS#6640
Other cross product results in this function could also be normalized but only the up vector was causing the effect noticed in the mentioned issue.

Source/Core/Transforms.js

@@ -765,6 +765,7 @@ define([
         }
 
         var up = Cartesian3.cross(right, velocity, upScratch);
+        Cartesian3.normalize(up, up);
         Cartesian3.cross(velocity, up, right);
         Cartesian3.negate(right, right);