Fix Bug #1336534 pcbnew 3D viewer wrong vertical model offset.

    // Init  Z position of each layer

    // calculate z position for each copper layer

    // Z = 0 is the z position of the back (bottom) layer (layer id = 31)

    // Z = m_EpoxyThickness is the z position of the front (top) layer (layer id = 0)

    // all unused copper layer z position are set to 0

    int layer;

    int copper_layers_cnt = m_CopperLayersCount;

    for( layer = 0; layer < copper_layers_cnt; layer++ )

    {

        m_LayerZcoord[layer] =

            m_EpoxyThickness * layer / (copper_layers_cnt - 1);

            m_EpoxyThickness - (m_EpoxyThickness * layer / (copper_layers_cnt - 1));

    }

    #define layerThicknessMargin 1.1

    double zpos_offset = m_NonCopperLayerThickness * layerThicknessMargin;

    double  zpos_copper_back    = m_LayerZcoord[0] - layerThicknessMargin*m_CopperThickness/2;

    double  zpos_copper_back    = - layerThicknessMargin*m_CopperThickness/2;

    double  zpos_copper_front   = m_EpoxyThickness + layerThicknessMargin*m_CopperThickness/2;

    // Fill remaining unused copper layers and front layer zpos

    // with m_EpoxyThickness

    // Solder mask and Solder paste have the same Z position

    // Fill remaining unused copper layers and back layer zpos

    // with 0

    for( ; layer < MAX_CU_LAYERS; layer++ )

    {

        m_LayerZcoord[layer] = m_EpoxyThickness;

        m_LayerZcoord[layer] = 0;

    }

    // calculate z position for each non copper layer

    // Solder mask and Solder paste have the same Z position

    for( int layer_id = MAX_CU_LAYERS; layer_id < LAYER_ID_COUNT; layer_id++ )

    {

        double zpos;