[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/src/clipboard.c

@@ -2116,6 +2116,35 @@ bool clipboard_requestReady(LG_ClipboardRequest id)
   return result;
 }
 
+bool clipboard_requestCancel(LG_ClipboardRequest id,
+    LG_ClipboardCancelReason reason)
+{
+  if (id == LG_CLIPBOARD_REQUEST_INVALID ||
+      (unsigned int)reason > LG_CLIPBOARD_CANCEL_INVALID ||
+      !clipboard.requests)
+    return false;
+
+  LG_LOCK(clipboard.callbackLock);
+  LG_LOCK(clipboard.requestLock);
+  ClipboardRequest * request = takeRequest(id);
+  LG_UNLOCK(clipboard.requestLock);
+  if (!request)
+  {
+    LG_UNLOCK(clipboard.callbackLock);
+    return false;
+  }
+
+  const ClipboardBinding binding = request->binding;
+  freeRequest(request);
+
+  LG_LOCK_SHARED(clipboard.activeLock);
+  if (bindingActiveNL(&binding) && binding.ops->requestCancel)
+    binding.ops->requestCancel(binding.opaque, id, reason);
+  LG_UNLOCK_SHARED(clipboard.activeLock);
+  LG_UNLOCK(clipboard.callbackLock);
+  return true;
+}
+
 bool clipboard_request(LG_ClipboardData type,
     LG_ClipboardReplyFn replyFn, void * opaque)
 {