weston-log: Return bytes written for 'printf()' and 'vprintf()' functions

Information is needed for 'vlog()' and 'vlog_continue()' (others
depend on them).

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

include/libweston/weston-log.h

@@ -79,11 +79,11 @@ void
 weston_log_scope_write(struct weston_log_scope *scope,
 			 const char *data, size_t len);
 
-void
+int
 weston_log_scope_vprintf(struct weston_log_scope *scope,
 			   const char *fmt, va_list ap);
 
-void
+int
 weston_log_scope_printf(struct weston_log_scope *scope,
 			  const char *fmt, ...)
 			  __attribute__ ((format (printf, 2, 3)));