[client] clipboard: keep controls responsive during stalls

Release LGMP stream slots before dispatching clipboard data so an X11
INCR consumer cannot prevent control queue acknowledgements.

Retain one bounded record in client memory while the sink is blocked.
Cancel abandoned X11 transfers after requestor destruction or a finite
progress timeout.

client/tests/x11_clipboard_test.c

@@ -586,6 +586,14 @@ bool clipboard_requestReady(LG_ClipboardRequest request)
   return true;
 }
 
+bool clipboard_requestCancel(LG_ClipboardRequest request,
+    LG_ClipboardCancelReason reason)
+{
+  CHECK(request == rec.requestId);
+  (void)reason;
+  return true;
+}
+
 bool clipboardFiles_setLocal(const char * mime,
     const void * data, size_t size)
 {