Fix to avoid excessive flushing of polygon batch.

It seems that when we use a starling SubTexture instead of an Spine texture, this line will trigger a lot of flushing instead of batching the polygons.
This fix avoid that.
Btw, you are using Animation.binarySearch1() in AnimationTimeline wich does not exists yet :-p
Merge pull request #303 from Fraggle/patch-3

Fix to avoid excessive flushing of polygon batch.