avconv: rewrite output data size tracking

Store a variable per OutputStream instead of globals for
audio/video/extradata. This makes the code simpler and cleaner and fixes
2pass with multiple output streams.

avconv.h

@@ -339,6 +339,10 @@ typedef struct OutputStream {
     enum AVPixelFormat pix_fmts[2];
 
     AVCodecParserContext *parser;
+
+    /* stats */
+    // combined size of all the packets written
+    uint64_t data_size;
 } OutputStream;
 
 typedef struct OutputFile {