ignore delta=0 mousewheel event

fix #1607

js/reveal.js

@@ -4297,7 +4297,9 @@
 			if( delta > 0 ) {
 				navigateNext();
 			}
-			else {
+			// fix #1607: On MacBook trackpad, a `delta=0` mousewheel event
+			// will be triggered as soon as two fingers touch.
+			else if( delta < 0 ) {
 				navigatePrev();
 			}