Support -startat and -endat for teletext input

Just a couple fix we added to have -startat and -endat works for teletext input.

Plus a couple bug fixes:
 * a typo from previous pull request in smptett headers (a missing '=')
 * -delay with large enough negative value would overflow
 * timeExpression should use . for the fraction part instead of , (cf [W3C Recommendation](http://www.w3.org/TR/ttaf1-dfxp/#timing-value-timeExpression))