avio: add avio_skip macro

This is a substitute for the url_fskip function that was deprecated by
commit 0300db8a. avio_fskip is provided to
improve demuxer code readability. It distinguishes the act of skipping over
unknown or irrelevant bytes from the standard avio_seek operation.
Signed-off-by: NRonald S. Bultje <rsbultje@gmail.com>

libavformat/avio.h

@@ -473,6 +473,12 @@ int avio_put_str16le(AVIOContext *s, const char *str);
  */
 int64_t avio_seek(AVIOContext *s, int64_t offset, int whence);
 
+/**
+ * Skip given number of bytes forward
+ * @return new position or AVERROR.
+ */
+#define avio_skip(s, offset) avio_seek(s, offset, SEEK_CUR)
+
 /**
  * ftell() equivalent for AVIOContext.
  * @return position or AVERROR.