Parametric Center Bed Support Customizable

The center of my heat bed was lower than the rest - this took care of that.

Use this to generate a center bed support model of any height which mounts between the y-carriage and bed, using the mount points of the y-belt holder.

You can then adjust the height at the four corners of the bed to raise or lower the center support relative to the corners.

Ironically, I have no idea why only the height setting doesn't work in the customizer web app. It works fine in OpenSCAD. Maybe someone ca