Clear clipboard state when its not plain text

Commit 8a80c90 added a check to prevent
announcing clipboard when it's not plain text data. When this check
triggers and we want to ignore the non-plain text clipboard, we should
reset the state by no longer indicating pending clipboard, and by asking
the server to clear its clipboard. This ensures we don't end up in a
state where the server thinks the viewer has useful clipboard data, but
it doesn't.

vncviewer/Viewport.cxx

@@ -756,6 +756,9 @@ void Viewport::handleClipboardChange(int source, void *data)
 
   if (!Fl::clipboard_contains(Fl::clipboard_plain_text)) {
     vlog.debug("Got non-plain text in local clipboard, ignoring.");
+    // Reset the state as if we don't have any clipboard data at all
+    self->pendingClientClipboard = false;
+    self->cc->announceClipboard(false);
     return;
   }