Replace deprecated function wlc_output_get_pixels.

This makes IPC GET_PIXELS use the new `wlc_pixels_read` call instead of
the deprecated `wlc_output_get_pixels`.

The old version worked by passing a callback function to wlc which would
grab the pixels and send them to the IPC client.
The new version works by maintaining a list of clients who have
requested the pixels of some output and then grap and send the pixels in
the output_post_render hook of the `wlc_interface`.

include/ipc-server.h

@@ -1,6 +1,8 @@
 #ifndef _SWAY_IPC_SERVER_H
 #define _SWAY_IPC_SERVER_H
 
+#include <wlc/wlc.h>
+
 #include "container.h"
 #include "config.h"
 #include "ipc.h"
@@ -27,4 +29,9 @@ void ipc_event_modifier(uint32_t modifier, const char *state);
 void ipc_event_binding_keyboard(struct sway_binding *sb);
 const char *swayc_type_string(enum swayc_types type);
 
+/**
+ * Send pixel data to registered clients.
+ */
+void ipc_get_pixels(wlc_handle output);
+
 #endif