Correctly mask non-BMP clipboard characters

JavaScript strings use UTF-16 encoding under the hood, but we only want
a single '?' per character we replace. So we need to be more careful
which methods we use when iterating over the clipboard string.

core/rfb.js

@@ -490,16 +490,27 @@ export default class RFB extends EventTargetMixin {
             this._clipboardText = text;
             RFB.messages.extendedClipboardNotify(this._sock, [extendedClipboardFormatText]);
         } else {
-            let data = new Uint8Array(text.length);
+            let length, i;
-            for (let i = 0; i < text.length; i++) {
+            let data;
-                let code = text.charCodeAt(i);
+
+            length = 0;
+            // eslint-disable-next-line no-unused-vars
+            for (let codePoint of text) {
+                length++;
+            }
+
+            data = new Uint8Array(length);
+
+            i = 0;
+            for (let codePoint of text) {
+                let code = codePoint.codePointAt(0);
 
                 /* Only ISO 8859-1 is supported */
                 if (code > 0xff) {
                     code = 0x3f; // '?'
                 }
 
-                data[i] = code;
+                data[i++] = code;
             }
 
             RFB.messages.clientCutText(this._sock, data);

tests/test.rfb.js

@@ -441,6 +441,14 @@ describe('Remote Frame Buffer Protocol Client', function () {
                                                                                new Uint8Array([97, 98, 99, 63]));
                 });
 
+                it('should mask characters, not UTF-16 code points', function () {
+                    client.clipboardPasteFrom('😂');
+
+                    expect(RFB.messages.clientCutText).to.have.been.calledOnce;
+                    expect(RFB.messages.clientCutText).to.have.been.calledWith(client._sock,
+                                                                               new Uint8Array([63]));
+                });
+
                 it('should send an notify if extended clipboard is supported by server', function () {
                     // Send our capabilities
                     let data = [3, 0, 0, 0];