image2: Add "start_number" private option to the muxer

This adds the capability to start counting file number from an arbitrary
integer instead of always starting at 1.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/img2enc.c

@@ -42,7 +42,6 @@ static int write_header(AVFormatContext *s)
     VideoMuxData *img = s->priv_data;
     const char *str;
 
-    img->img_number = 1;
     av_strlcpy(img->path, s->filename, sizeof(img->path));
 
     /* find format */
@@ -137,6 +136,7 @@ static int write_packet(AVFormatContext *s, AVPacket *pkt)
 #define ENC AV_OPT_FLAG_ENCODING_PARAM
 static const AVOption muxoptions[] = {
     { "updatefirst",  "", OFFSET(updatefirst),  AV_OPT_TYPE_INT,    {.dbl = 0},    0, 1, ENC },
+    { "start_number", "first number in the sequence", OFFSET(img_number), AV_OPT_TYPE_INT, {.dbl = 1}, 1, INT_MAX, ENC },
     { NULL },
 };