fix(panel): _md-panel-offscreen not removed from innerWrapper

- this resulted in animated in panels not displaying on screen

Fixes #11946

Author: Splaktar

package-lock.json

@@ -2023,6 +2023,7 @@ MdPanelRef.prototype._updatePosition = function(init) {
     // Hide the panel now that position is known.
     if (init) {
       this.panelEl.removeClass('_md-panel-offscreen');
+      this.innerWrapper.removeClass('_md-panel-offscreen');
       this.panelContainer.addClass(MD_PANEL_HIDDEN);
     }