`Track_on` with 0 offset does not trigger `Track_starts`
See we need to put 1 tick there: https://gitlab.com/smondet/vecosek/blob/c845c20416c05dcccc55dcec9d5bd17a11671e90/src/test/scenes.ml#L170-172
See we need to put 1 tick there: https://gitlab.com/smondet/vecosek/blob/c845c20416c05dcccc55dcec9d5bd17a11671e90/src/test/scenes.ml#L170-172