fix camera behavior with upsideDown mode

dist/preview release/babylon.max.js

@@ -18716,7 +18716,7 @@ var BABYLON;
             else {
                 this.position.copyFrom(this._newPosition);
                 var up = this.upVector;
-                if (this.allowUpsideDown && this.beta < 0) {
+                if (this.allowUpsideDown && sinb < 0) {
                     up = up.clone();
                     up = up.negate();
                 }

src/Cameras/babylon.arcRotateCamera.ts

@@ -445,7 +445,7 @@ module BABYLON {
                 this.position.copyFrom(this._newPosition);
 
                 var up = this.upVector;
-                if (this.allowUpsideDown && this.beta < 0) {
+                if (this.allowUpsideDown && sinb < 0) {
                     up = up.clone();
                     up = up.negate();
                 }