avformat/avio: privatize source of truth for AVIOContext::written

Looking at 3f75e5116b, the field
was supposed to be private, but during merging the field and the
group that had the comment about it got separated.

Thus, move the actual privately utilized state of this variable
into the private FFIOContext. Additionally, name the private field
somewhat better, so that it does not get confused with the amount
of bytes written out.

libavformat/avio_internal.h

@@ -66,6 +66,12 @@ typedef struct FFIOContext {
      * used after probing to ensure seekback and to reset the buffer size
      */
     int orig_buffer_size;
+
+    /**
+     * Written output size
+     * is updated each time a successful writeout ends up further position-wise
+     */
+    int64_t written_output_size;
 } FFIOContext;
 
 static av_always_inline FFIOContext *ffiocontext(AVIOContext *ctx)