Add backend_info_t::max_buffer_age

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

src/backend/backend.h

@@ -143,6 +143,9 @@ typedef struct backend_info {
 	/// buffer is empty.
 	int (*buffer_age)(void *backend_data, session_t *);
 
+	/// The maximum number buffer_age might return.
+	int max_buffer_age;
+
 	// ===========         Hooks        ============
 	/// Let the backend hook into the event handling queue
 } backend_info_t;

src/backend/gl/glx.c

@@ -604,4 +604,5 @@ backend_info_t glx_backend = {
     .present = glx_present,
     .is_win_transparent = default_is_win_transparent,
     .is_frame_transparent = default_is_frame_transparent,
+    .max_buffer_age = 5,        // XXX why?
 };

src/backend/xrender.c

@@ -503,6 +503,7 @@ struct backend_info xrender_backend = {
     .release_win = release_win,
     .is_win_transparent = default_is_win_transparent,
     .is_frame_transparent = default_is_frame_transparent,
+    .max_buffer_age = 2,
 };
 
 // vim: set noet sw=8 ts=8: