Fix initial padding and startPosition when scrolling is delayed

[emagnier]

When you add a startPadding and use the startAfter to delay the scrolling after 'n' seconds: on this few 'n' seconds the startPadding is not applied (internally, the startPosition is not correctly set).

Below, on the first 5 seconds, you can see that the startPadding is ignored, and cause a "jump" effect when it starts scrolling:

Marquee(
  text: 'Lorem ipsum dolor sit amet, consectetur adipiscing elit.',
  scrollAxis: Axis.horizontal,
  crossAxisAlignment: CrossAxisAlignment.start,
  blankSpace: 100.0,
  showFadingOnlyWhenScrolling: false,
  fadingEdgeStartFraction: 0.1,
  fadingEdgeEndFraction: 0.1,
  startPadding: 15,
  startAfter: Duration(seconds: 3),
  pauseAfterRound: Duration(seconds: 5),
),

This fix here ensures that the startPosition is correctly set from the initState of this Widget.

[MarcelGarus]

Published this as 2.1.0. Thanks for contributing!

[emagnier]

Great! Thank you.