[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/include/core/clipboard.h

@@ -53,6 +53,11 @@ bool clipboard_requestStream(LG_ClipboardData type,
     const LG_ClipboardStreamOps * stream, void * opaque,
     LG_ClipboardRequest * request);
 bool clipboard_requestReady(LG_ClipboardRequest request);
+/* The consumer must unlink its stream state before cancellation, but retain
+ * its opaque storage until this returns. Success means the local request was
+ * canceled; delivery to the provider is best effort. */
+bool clipboard_requestCancel(LG_ClipboardRequest request,
+    LG_ClipboardCancelReason reason);
 bool clipboard_request(LG_ClipboardData type,
     LG_ClipboardReplyFn replyFn, void * opaque);