#1128 MultiplePieChart3D with gradients, colors not being centered correctly

[sandstones]

MultiplePieChart3D used with a gradient paint centers the gradient wrt the entire plot rather than wrt each individual pie.

Fix was to have PiePlot3D request the gradient for a particular pie plot state, rather than for the entire pie plot. Minor changes to PiePlot and PiePlot3D - patch file attached.

See attached image with MultiplePieChart3D with gradients before and after fix.

[sandstones]

Patch file attached here