title: Background image on relative layout don't get the screen size on forms
body: <p>This is the code of the image and layout from a tabbed page:</p>
  
  <pre><code>RelativeLayout PrincipalLayout = new RelativeLayout ();
  
                  Image background = new Image {
                      Source = Images.Background,
                  };
  
                  PrincipalLayout.Children.Add (background,
                      xConstraint: Constraint.RelativeToParent ((parent) =&gt; {
                          return 0;
                      }),
                      yConstraint: Constraint.RelativeToParent ((parent) =&gt; {
                          return 0;
                      }),
                      widthConstraint: Constraint.RelativeToParent ((parent) =&gt; {
                          return parent.Width;
                      }),
                      heightConstraint: Constraint.RelativeToParent ((parent) =&gt; {
                          return parent.Height;
                      })
                  );
  this.Content = PrincipalLayout;
  </code></pre>
  
  <p>But the image don't start at X = 0 and don't finish at the right place.</p>
  
  <p>Take a look at the image:</p>
  
  <p><a href="http://i292.photobucket.com/albums/mm15/romuloviel/Captura%20de%20Tela%202015-03-18%20as%2010.06.34_zpsb4cquhtd.png" rel="nofollow">http://i292.photobucket.com/albums/mm15/romuloviel/Captura%20de%20Tela%202015-03-18%20as%2010.06.34_zpsb4cquhtd.png</a></p>
  
  <p>I need the image get all frame.</p>