fix when add layer cause page forced reflow

see [swisnl#721](swisnl#721)

Author: Rhain

src/jquery.contextMenu.js

@@ -1505,15 +1505,15 @@
                         width: $win.width(),
                         display: 'block',
                         position: 'fixed',
-                        'z-index': zIndex,
+                        'z-index': zIndex - 1,
                         top: 0,
                         left: 0,
                         opacity: 0,
                         filter: 'alpha(opacity=0)',
                         'background-color': '#000'
                     })
                     .data('contextMenuRoot', opt)
-                    .insertBefore(this)
+                    .appendTo(document.body)
                     .on('contextmenu', handle.abortevent)
                     .on('mousedown', handle.layerClick);