Add warning about screen sharing under Safari.

static/sfu.js

@@ -818,6 +818,8 @@ async function addLocalMedia(id) {
     setButtonsVisibility();
 }
 
+let safariWarningDone = false;
+
 async function addShareMedia() {
     if(!getUserPass())
         return;
@@ -835,6 +837,15 @@ async function addShareMedia() {
         return;
     }
 
+    if(!safariWarningDone) {
+        let ua = navigator.userAgent.toLowerCase();
+        if(ua.indexOf('safari') >= 0 && ua.indexOf('chrome') < 0) {
+            displayWarning('Screen sharing under Safari is experimental.  ' +
+                           'Please use a different browser if possible.');
+        }
+        safariWarningDone = true;
+    }
+
     let c = newUpStream();
     c.kind = 'screenshare';
     c.stream = stream;