plugins/zoom: Center zoom area around middle of workspace in Push mode

This merge request changes how the Push tracking mode calculates the zoom viewport. Rather than using math from the Proportional mode, it uses the math from Centered/Disabled modes. This allows me to more easily pan the zoom area toward the mouse cursor rather than to some arbitrary point on the desktop.

I no longer have multiple monitors to test on. However, my understanding is they shouldn't be affected by this change. Could anyone with more than one screen help me out with making sure?

The idea behind this change is I find it very easy to lose my place when going from fully zoomed out to zoomed in on some parts of my screen. Unless I go slowly, it's easy to lose the mouse cursor because the zoom area eventually pans away from it.