dashenc: use avio_dynbuf instead of packet_write callback

The dash_write function drops data, if no IOContext is initialized.

Since the mp4 muxer is used in "frag_custom" mode, data is only
written when calling av_write_frame(NULL) explicitly and thus
there will be no data loss.

To add support for webm as subordinate muxer, which doesn't have
such a mode, a dynamic buffer is required to provide an always
initialized IOContext.

Signed-off-by: Peter Große <pegro@friiks.de>
Signed-off-by: Martin Storsjö <martin@martin.st>