[gbrainy] Fix background in newever versions of cairo

[Jordi Mas <jmas src gnome org>]

commit ee82e900ce4034679d7f8cbf219307ffcf2e039e
Author: Jordi Mas <jmas softcatala org>
Date:   Sun Sep 23 07:38:32 2018 +0200

    Fix background in newever versions of cairo

 src/Core/Main/CairoContextEx.cs | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/src/Core/Main/CairoContextEx.cs b/src/Core/Main/CairoContextEx.cs
index f5db265..6299644 100644
--- a/src/Core/Main/CairoContextEx.cs
+++ b/src/Core/Main/CairoContextEx.cs
@@ -71,7 +71,8 @@ namespace gbrainy.Core.Main
 
                                Save ();
                                Rectangle (0, 0, 1, 1);
-                               Scale (0.999 / image.Width, 0.999 / image.Height);
+                Stroke ();
+                               Scale (0.999999 / image.Width, 0.999999 / image.Height);
                                image.RenderToCairo (Handle);
                                Restore ();
                        }