The Materials module does a conversion from float to double when saving the transparency and again a conversion from double to float to double
when restoring it. This causes a considerable loss of accuracy so that the representation in percent leads to different numbers.
Using consistently some helper functions to do a proper conversion from float to long and back fixes the problem.
The new property ShapeAppearance is a PropertyMaterialList and always read after the DiffuseColor property when restoring a document.
Thus, the method onChanged() doesn't override DiffuseColor when restoring a document. Additionally, the method finishRestoring() is re-implemented
to set the colours per face in case DiffuseColor has defined multiple colors.
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