Markers added at the start of the timeline need to be special cased
because we effectively check whether the marker time is greater than
the old frame time or less than or equal to the new frame time but we
never actually emit a frame for the start of the timeline.
If the timeline is looping then it adjusts the position to interpolate
a wrapped around position. However we do not emit a new frame after
setting this position so we need to check for markers again there.
clutter_timeline_get_delta should return the actual wall clock time
between emissions of the new-frame signal - even if the elapsed time
has been fudged at the end of the timeline. To make this work it no
longer directly manipulates priv->msecs_delta but instead passes a
delta value to check_markers.
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