Don't allow drawing to a zero-height or zero-width GAL canvas.

Fixes: lp:1793913
* https://bugs.launchpad.net/kicad/+bug/1793913

common/draw_panel_gal.cpp

@@ -147,6 +147,12 @@ void EDA_DRAW_PANEL_GAL::SetFocus()
 
 void EDA_DRAW_PANEL_GAL::onPaint( wxPaintEvent& WXUNUSED( aEvent ) )
 {
+    m_viewControls->UpdateScrollbars();
+
+    // Drawing to a zero-width or zero-height GAL is fraught with peril.
+    if( GetClientRect().IsEmpty() )
+        return;
+
     m_pendingRefresh = false;
 
     if( m_drawing )
@@ -161,8 +167,6 @@ void EDA_DRAW_PANEL_GAL::onPaint( wxPaintEvent& WXUNUSED( aEvent ) )
     m_drawing = true;
     KIGFX::RENDER_SETTINGS* settings = static_cast<KIGFX::RENDER_SETTINGS*>( m_painter->GetSettings() );
 
-    m_viewControls->UpdateScrollbars();
-
     try
     {
         m_view->UpdateItems();

common/gal/opengl/opengl_gal.cpp

@@ -91,6 +91,7 @@ private:
 
 };
 
+
 GL_BITMAP_CACHE::~GL_BITMAP_CACHE()
 {
     for ( auto b = m_bitmaps.begin(); b != m_bitmaps.end(); ++b )
@@ -99,6 +100,7 @@ GL_BITMAP_CACHE::~GL_BITMAP_CACHE()
     }
 }
 
+
 GLuint GL_BITMAP_CACHE::RequestBitmap( const BITMAP_BASE* aBitmap )
 {
     auto it = m_bitmaps.find( aBitmap) ;
@@ -320,7 +322,7 @@ bool OPENGL_GAL::updatedGalDisplayOptions( const GAL_DISPLAY_OPTIONS& aOptions )
 
 void OPENGL_GAL::BeginDrawing()
 {
-    if( !IsShownOnScreen() )
+    if( !IsShownOnScreen() || GetClientRect().IsEmpty() )
         return;
 
 #ifdef __WXDEBUG__
@@ -491,7 +493,7 @@ void OPENGL_GAL::EndDrawing()
 
 void OPENGL_GAL::BeginUpdate()
 {
-    if( !IsShownOnScreen() )
+    if( !IsShownOnScreen() || GetClientRect().IsEmpty() )
         return;
 
     if( !isInitialized )

include/gal/cairo/cairo_gal.h

@@ -88,8 +88,9 @@ public:
     virtual ~CAIRO_GAL();
 
     ///> @copydoc GAL::IsVisible()
-    bool IsVisible() const override {
-        return IsShownOnScreen();
+    bool IsVisible() const override
+    {
+        return IsShownOnScreen() && !GetClientRect().IsEmpty();
     }
 
     // ---------------

include/gal/opengl/opengl_gal.h

@@ -90,13 +90,13 @@ public:
     virtual bool IsInitialized() const override
     {
         // is*Initialized flags, but it is enough for OpenGL to show up
-        return IsShownOnScreen();
+        return IsShownOnScreen() && !GetClientRect().IsEmpty();
     }
 
     ///> @copydoc GAL::IsVisible()
     bool IsVisible() const override
     {
-        return IsShownOnScreen();
+        return IsShownOnScreen() && !GetClientRect().IsEmpty();
     }
 
     // ---------------