More robust checks for OpenGL canvas visibility.

common/draw_panel_gal.cpp

@@ -479,7 +479,7 @@ void EDA_DRAW_PANEL_GAL::onRefreshTimer( wxTimerEvent& aEvent )
 
 void EDA_DRAW_PANEL_GAL::onShowTimer( wxTimerEvent& aEvent )
 {
-    if( IsShownOnScreen() )
+    if( m_gal && m_gal->IsVisible() )
     {
         m_onShowTimer.Stop();
         OnShow();

include/gal/cairo/cairo_gal.h

@@ -85,6 +85,11 @@ public:
 
     virtual ~CAIRO_GAL();
 
+    ///> @copydoc GAL::IsVisible()
+    bool IsVisible() const override {
+        return IsShownOnScreen();
+    }
+
     // ---------------
     // Drawing methods
     // ---------------

include/gal/graphics_abstraction_layer.h

@@ -69,6 +69,9 @@ public:
     /// @brief Returns the initalization status for the canvas.
     virtual bool IsInitialized() const { return true; }
 
+    /// @brief Returns true if the GAL canvas is visible on the screen.
+    virtual bool IsVisible() const { return true; }
+
     // ---------------
     // Drawing methods
     // ---------------

include/gal/opengl/opengl_gal.h

@@ -86,6 +86,11 @@ public:
     /// @copydoc GAL::IsInitialized()
     virtual bool IsInitialized() const { return IsShownOnScreen(); }
 
+    ///> @copydoc GAL::IsVisible()
+    bool IsVisible() const override {
+        return IsShownOnScreen();
+    }
+
     // ---------------
     // Drawing methods
     // ---------------