Fixes missing worksheet layout when there is no PCB loaded and all random...

Fixes missing worksheet layout when there is no PCB loaded and all random brightness changes for the Cairo rendering backend.

common/gal/cairo/cairo_gal.cpp

@@ -35,6 +35,9 @@
 
 using namespace KIGFX;
 
+///> Opacity of a single layer
+const float LAYER_ALPHA = 0.8;
+
 CAIRO_GAL::CAIRO_GAL( wxWindow* aParent, wxEvtHandler* aMouseListener,
         wxEvtHandler* aPaintListener, const wxString& aName ) :
     wxWindow( aParent, wxID_ANY, wxDefaultPosition, wxDefaultSize, wxEXPAND, aName )
@@ -111,7 +114,7 @@ void CAIRO_GAL::EndDrawing()
 
     // Cairo grouping prevents display of overlapping items on the same layer in the lighter color
     cairo_pop_group_to_source( currentContext );
-    cairo_paint_with_alpha( currentContext, fillColor.a );
+    cairo_paint_with_alpha( currentContext, LAYER_ALPHA );
 
     // Merge buffers on the screen
     compositor->DrawBuffer( mainBuffer );
@@ -417,7 +420,7 @@ void CAIRO_GAL::SetLayerDepth( double aLayerDepth )
         storePath();
 
         cairo_pop_group_to_source( currentContext );
-        cairo_paint_with_alpha( currentContext, fillColor.a );
+        cairo_paint_with_alpha( currentContext, LAYER_ALPHA );
 
         cairo_push_group( currentContext );
     }
@@ -743,7 +746,7 @@ void CAIRO_GAL::SetTarget( RENDER_TARGET aTarget )
         storePath();
 
         cairo_pop_group_to_source( currentContext );
-        cairo_paint_with_alpha( currentContext, fillColor.a );
+        cairo_paint_with_alpha( currentContext, LAYER_ALPHA );
     }
 
     switch( aTarget )