Add flag so muxers not needing width/height can signal this.

Add this flag to img2 (fixes -vcodec copy to image2 in some cases)

Originally committed as revision 21773 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/avformat.h

@@ -242,6 +242,7 @@ typedef struct AVFormatParameters {
 #define AVFMT_GENERIC_INDEX 0x0100 /**< Use generic index building code. */
 #define AVFMT_TS_DISCONT    0x0200 /**< Format allows timestamp discontinuities. */
 #define AVFMT_VARIABLE_FPS  0x0400 /**< Format allows variable fps. */
+#define AVFMT_NODIMENSIONS  0x0800 /**< Format does not need width/height */
 
 typedef struct AVOutputFormat {
     const char *name;