#7 fixes to use the real center of VV

[Sergio Guzman]

I found that after changing the translate values (with
TranslatingGraphMousePlugin) and the scale (with
CrossoverScalingGraphMousePlugin) several times, the
center of the screen gets a little lost, so the
animation moves the picked vertex to another place that
is not the VV.getCenter()

Here is the patch to solve this situation.

[Sergio Guzman]

Patch to get the real center of screen

[Sergio Guzman]

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Duh!! sorry I didn't realize that the patch was never uploaded