Shapes 3D effects can specify a material like in the following example:

  <a:sp3d prstMaterial="metal" z="488950" />

This patch preserves the prstMaterial attribute in the sp3d tag using
the shape grab bag and modifies an existing unit test to add this
check.

Change-Id: I7be2dbbcc7e599d5f0fb8fa53ec1d180c18d8ebd