debug: Print debug message when client wakes up

This helps spot and remove extraneous round trips and context switches.

src/wayland-client.c

diff --git a/src/wayland-client.c b/src/wayland-client.c
index de84698..efb7fe6 100644
--- a/src/wayland-client.c
+++ b/src/wayland-client.c
@@ -450,6 +450,11 @@
 	}
 
 	len = wl_connection_data(display->connection, mask);
+
+	if (wl_debug && (mask & WL_DISPLAY_READABLE))
+		fprintf(stderr,
+			"[-----------] wakeup, read %d bytes\n", len);
+
 	while (len > 0) {
 		if (len < sizeof p)
 			break;