compositor: Screenhoot into a client provided shm buffer

This moves the png writing part to the client and removes the gdk-pixbuf
dependency from the compositor.
diff --git a/compositor/compositor.c b/compositor/compositor.c
index 987267e..a295991 100644
--- a/compositor/compositor.c
+++ b/compositor/compositor.c
@@ -29,11 +29,11 @@
 #include <sys/ioctl.h>
 #include <fcntl.h>
 #include <unistd.h>
-#include <gdk-pixbuf/gdk-pixbuf.h>
 #include <math.h>
 #include <linux/input.h>
 #include <dlfcn.h>
 #include <getopt.h>
+#include <signal.h>
 
 #include "wayland-server.h"
 #include "compositor.h"
@@ -1883,8 +1883,6 @@
 		{ NULL, }
 	};
 
-	g_type_init(); /* GdkPixbuf needs this, it seems. */
-
 	width = 1024;
 	height = 640;