avutil/frame: deprecate palette_has_changed

Not only this is information that relies on the concept of a sequence of
frames, which is completely out of place as a field in AVFrame, but there are
no known or intended uses of this field.

Signed-off-by: James Almer <jamrial@gmail.com>

libavutil/frame.h

@@ -528,10 +528,13 @@ typedef struct AVFrame {
     int top_field_first;
 #endif
 
+#if FF_API_PALETTE_HAS_CHANGED
     /**
      * Tell user application that palette has changed from previous frame.
      */
+    attribute_deprecated
     int palette_has_changed;
+#endif
 
 #if FF_API_REORDERED_OPAQUE
     /**