client: Add an entry point for dispatching a queue without blocking

On the client side EGL, all the wl_buffer.release events need to be
processed before buffer allocation, otherwise a third buffer might
be allocated unnecessarily. However, the buffer allocation should
not block in the case no event was received. In order to do that, a
non-blocking queue dispatch function is needed.

src/wayland-client.c

@@ -926,6 +926,25 @@ wl_display_dispatch_queue(struct wl_display *display,
 	return dispatch_queue(display, queue, 1);
 }
 
+/** Dispatch pending events in an event queue
+ *
+ * \param display The display context object
+ * \param queue The event queue to dispatch
+ * \return The number of dispatched events on success or -1 on failure
+ *
+ * Dispatch all incoming events for objects assigned to the given
+ * event queue. On failure -1 is returned and errno set appropriately.
+ * If there are no events queued, this functions return immediately.
+ *
+ * \memberof wl_display
+ */
+WL_EXPORT int
+wl_display_dispatch_queue_pending(struct wl_display *display,
+				  struct wl_event_queue *queue)
+{
+	return dispatch_queue(display, queue, 0);
+}
+
 /** Process incoming events
  *
  * \param display The display context object

src/wayland-client.h

@@ -145,6 +145,8 @@ int wl_display_get_fd(struct wl_display *display);
 int wl_display_dispatch(struct wl_display *display);
 int wl_display_dispatch_queue(struct wl_display *display,
 			      struct wl_event_queue *queue);
+int wl_display_dispatch_queue_pending(struct wl_display *display,
+				      struct wl_event_queue *queue);
 int wl_display_dispatch_pending(struct wl_display *display);
 int wl_display_get_error(struct wl_display *display);