timeline: Cast elapsed time before emitting ::new-frame

timeline: Cast elapsed time before emitting ::new-frame

When emitting a new-frame signal, priv->elapsed_time is passed as a
parameter. This is a gint64. The closure marshal uses an INT. On some
platforms, this is not received correctly by signal handlers (they
receive 0). One solution is to cast priv->elapsed_time to a gint when
emitting the signal.

We cannot change the signature of the signal without breaking ABI.

https://bugzilla.gnome.org/show_bug.cgi?id=654066