Fix some problems in new backend integration

Fix several use of uninitialized/unallocated variables when using
--experimental-backends

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/render.h

@@ -42,3 +42,5 @@ void free_root_tile(session_t *ps);
 
 bool init_render(session_t *ps);
 void deinit_render(session_t *ps);
+
+int maximum_buffer_age(session_t *);