avfilter: add needs_writable field to the internal AVFilterPad structure

libavfilter/internal.h

@@ -117,6 +117,14 @@ struct AVFilterPad {
      * input pads only.
      */
     int needs_fifo;
+
+    /**
+     * The filter expects writable frames from its input link,
+     * duplicating data buffers if needed.
+     *
+     * input pads only.
+     */
+    int needs_writable;
 };
 #endif