dashdemux: support ISO8601 durations that overflow
ISO-8601 durations can contain hours, minutes and seconds, each of which is optional. Each of these can be an unbounded number and are not limited to 24 hours, 60 minutes or 60 seconds.
gstmpdparser was making the assumption that there was a limit on the size of hours, minutes and seconds, which means that it is unable to parse a DASH manifest that makes use of this feature of ISO8601.
Fixes #840 (moved)