Gate open and close animation transition

[Idanwin]

When a unit comes close to a gate, that gate opens. When the unit does not advance further towards the gate but instead leaves that zone immediately the gate will jump brusquely from its opening animation to its closing animation. Proposed solutions:
1) The gate-open animation should be allowed to finish before the gate-close animation is started.
2) The gate-close animation should start were the gate-open animation was left off.

[historic_bruno]

Agreed, this is a duplicate of #1550 though. The problem is the animation system isn't flexible enough yet for that fine grained control.