GtkTimeline: Wrap around the progress correctly when looping

GtkTimeline: Wrap around the progress correctly when looping

When we have a looping animation for something like an angle,
we need to make sure that the distance we go past 1.0 becomes
the starting distance for the next frame. This prevents a
stutter at the loop position.

https://bugzilla.gnome.org/show_bug.cgi?id=685460