Wayland: logging

The core libwayland libraries should not handle logging, only passing
the error messages to subscribed functions.
An application linked to libwayland-server or libwayland-client
will be able to set own functions (one per library) to handle error
messages.

Change in this series: make the wl_log return int, because
of compatibility with printf. It will return the number of bytes logged.

src/wayland-server.h

@@ -418,6 +418,8 @@ wl_buffer_is_shm(struct wl_buffer *buffer);
 int
 wl_display_init_shm(struct wl_display *display);
 
+void wl_log_set_handler_server(wl_log_func_t handler);
+
 #ifdef  __cplusplus
 }
 #endif