StdPrs_ShadedShape, add flag theToExploreSolids to method ::Add().
Previously the tool collected triagnulations from all Faces into single graphic group.
The automated algorithm for back-face culling activation has to disable culling
when within closed Solids single Compound contains also open Shells.
Now tool is able to optionally (enabled by default) split these two categories of primitives
into dedicated groups with independent back-face culling settings.
In addition, this closed flag is now stored in Graphic3d_Group::myIsClosed which allows
capping algorithm OpenGl_CappingAlgo to automatically filter improper groups.
