make slide present mode exit button clickable at all sizes

customize.dist/main.css

@@ -1294,6 +1294,7 @@ html.cp,
 .cp.slide #modal #button_exit {
   left: 20px;
   top: 20px;
+  z-index: 9001;
 }
 .cp.slide #modal #button_left {
   left: 6vw;

customize.dist/src/less/cryptpad.less

@@ -848,6 +848,7 @@ form.realtime, div.realtime {
         #button_exit {
             left: 20px;
             top: 20px;
+            z-index: 9001;
         }
         #button_left {
             left: 6vw;