[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/interface/clipboard.h

@@ -236,6 +236,10 @@ typedef struct LG_ClipboardOps
    * or release first. */
   bool (*request)(void * opaque, LG_ClipboardRequest request,
       LG_ClipboardData type);
+  /* Best-effort cancellation of a request previously submitted through
+   * request(). The consumer has already made its stream state inactive. */
+  bool (*requestCancel)(void * opaque, LG_ClipboardRequest request,
+      LG_ClipboardCancelReason reason);
 
   bool (*fileAcquire)(void * opaque, uint64_t dataset,
       uint64_t acquisition);