win: update window's image lazily

Use a flag to mark whether the image needs update, and only update once
per frame.

Also refactor some common code into functions, and handle image errors
properly.

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

src/win.h

@@ -18,6 +18,7 @@
 #include "render.h"
 #include "types.h"
 #include "utils.h"
+#include "backend/backend.h"
 #include "x.h"
 
 typedef struct session session_t;
@@ -104,6 +105,13 @@ typedef enum {
 	WSTATE_UNMAPPED,
 } winstate_t;
 
+typedef enum win_flags {
+	/// win_image/shadow_image is out of date
+	WIN_FLAGS_STALE_IMAGE = 1,
+	/// there was an error trying to bind the images
+	WIN_FLAGS_IMAGE_ERROR = 2,
+} win_flags_t;
+
 /**
  * About coordinate systems
  *
@@ -282,6 +290,11 @@ struct win {
 #endif
 };
 
+void win_release_image(backend_t *base, win *w);
+bool must_use win_bind_image(session_t *ps, win *w);
+
+/// Attempt a rebind of window's images. If that failed, the original images are kept.
+bool win_try_rebind_image(session_t *ps, win *w);
 int win_get_name(session_t *ps, win *w);
 int win_get_role(session_t *ps, win *w);
 winmode_t attr_pure win_calc_mode(const win *w);