Change clippingDisplay() to a property

It fits much better as a property given that it only tells what the
current state of things are.

core/rfb.js

@@ -268,6 +268,8 @@ RFB.prototype = {
     get viewport() { return this._display.viewport; },
     set viewport(viewport) { this._display.viewport = viewport; },
 
+    get isClipped() { return this._display.isClipped; },
+
     // ===== EVENT HANDLERS =====
 
     onupdatestate: function () {},  // onupdatestate(rfb, state, oldstate): connection state change
@@ -380,11 +382,6 @@ RFB.prototype = {
         this._display.viewportChangeSize(width, height);
     },
 
-    clippingDisplay: function () {
-        if (this._rfb_connection_state !== 'connected') { return false; }
-        return this._display.clippingDisplay();
-    },
-
     // Requests a change of remote desktop size. This message is an extension
     // and may only be sent if we have received an ExtendedDesktopSize message
     requestDesktopSize: function (width, height) {