[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/transports/SPICE/clipboard.c

@@ -284,6 +284,21 @@ static bool spiceRequest(void * opaque, LG_ClipboardRequest request,
   return false;
 }
 
+static bool spiceRequestCancel(void * opaque,
+    LG_ClipboardRequest request, LG_ClipboardCancelReason reason)
+{
+  SpiceClipboard * clipboard = opaque;
+  (void)reason;
+
+  LG_LOCK(clipboard->stateLock);
+  const bool valid = clipboard->read.pending &&
+    clipboard->read.request == request;
+  if (valid)
+    clipboard->read = (PendingRequest) { 0 };
+  LG_UNLOCK(clipboard->stateLock);
+  return valid;
+}
+
 static const LG_ClipboardOps l_clipboardOps =
 {
   .name              = "SPICE",
@@ -294,6 +309,7 @@ static const LG_ClipboardOps l_clipboardOps =
   .notifyTypes       = spiceNotifyTypes,
   .data              = spiceData,
   .request           = spiceRequest,
+  .requestCancel     = spiceRequestCancel,
 };
 
 bool spiceClipboard_init(SpiceClipboard ** clipboard)