lavf: add AVFormatContext/AVStream fields for signaling to the user when events happen.

The only flags, for now, indicate if metadata was updated and are set after each call to
av_read_frame(). This comes with the caveat that, on stream start, it might not be set properly
as packets might be buffered in AVFormatContext.packet_buffer before being given to the user
in av_read_frame().

Signed-off-by: Anton Khirnov <anton@khirnov.net>