Made Context/RenderTarget usage constant

Former-commit-id: 07a2655ea642664bc49ca335cf5147ebf9fb9f26

include/Nazara/Renderer/RenderWindow.hpp

@@ -53,7 +53,7 @@ class NAZARA_API NzRenderWindow : public NzRenderTarget, public NzWindow
 		bool HasContext() const;
 
 	protected:
-		bool Activate() override;
+		bool Activate() const override;
 
 	private:
 		bool OnWindowCreated() override;