avformat: add url field to AVFormatContext

avformat: add url field to AVFormatContext

This will replace the 1024 character limited filename field. Compatiblity for
output contexts are provided by copying filename field to URL if URL is unset
and by providing an internal function for muxers to set both url and filename
at once.

Signed-off-by: Marton Balint <cus@passwd.hu>