Gerbview, GAL mode: fix "Zoom to fit" area size calculation when nothing is loaded.

Also ensure the worksheet is displayed after clearing the gerber layers.

gerbview/clear_gbr_drawlayers.cpp

@@ -55,6 +55,9 @@ bool GERBVIEW_FRAME::Clear_DrawLayers( bool query )
             m_toolManager->ResetTools( TOOL_BASE::MODEL_RELOAD );
 
         canvas->GetView()->Clear();
+
+        // Reinit the worksheet view, cleared by GetView()->Clear():
+        SetPageSettings( GetPageSettings() );
     }
 
     GetImagesList()->DeleteAllImages();

gerbview/gerbview_draw_panel_gal.cpp

@@ -168,3 +168,12 @@ void GERBVIEW_DRAW_PANEL_GAL::SetTopLayer( int aLayer )
 
     m_view->UpdateAllLayersOrder();
 }
+
+
+BOX2I GERBVIEW_DRAW_PANEL_GAL::GetDefaultViewBBox() const
+{
+    if( m_worksheet )
+        return m_worksheet->ViewBBox();
+
+    return BOX2I();
+}

gerbview/gerbview_draw_panel_gal.h

@@ -61,6 +61,9 @@ public:
     ///> @copydoc EDA_DRAW_PANEL_GAL::SetTopLayer
     virtual void SetTopLayer( int aLayer ) override;
 
+    ///> @copydoc EDA_DRAW_PANEL_GAL::GetDefaultViewBBox()
+    BOX2I GetDefaultViewBBox() const override;
+
     /**
      * Sets (or updates) worksheet used by the draw panel.
      * @param aWorksheet is the worksheet to be used.

gerbview/gerbview_frame.cpp

@@ -75,6 +75,10 @@ GERBVIEW_FRAME::GERBVIEW_FRAME( KIWAY* aKiway, wxWindow* aParent ):
                                              // obviously depends on the monitor,
                                              // but this is an acceptable value
 
+    // Be sure a page info is set. this default value will be overwritten later.
+    PAGE_INFO pageInfo( wxT( "GERBER" ) );
+    SetPageSettings( pageInfo );
+
     m_show_layer_manager_tools = true;
 
     m_showAxis = true;                      // true to show X and Y axis on screen