Fix OrthograficView::covers_viewport

src/map_view_gl.rs

 use mercator_tile_layer::MercatorTileLayer;
 use mercator_view::MercatorView;
 use ortho_tile_layer::OrthoTileLayer;
+use orthografic_view::OrthograficView;
 use projection::Projection;
 use session::Session;
 use texture::{Texture, TextureFormat};
     pub fn map_covers_viewport(&self) -> bool {
         match self.projection {
             Projection::Mercator => MercatorView::covers_viewport(&self.map_view),
-            //TODO uncomment
-            //Projection::Orthografic => OrthograficView::covers_viewport(&self.map_view),
-            Projection::Orthografic => false,
+            Projection::Orthografic => OrthograficView::covers_viewport(&self.map_view),
         }
     }
 

src/orthografic_view.rs

 impl OrthograficView {
     /// Returns true if the rendering covers the whole viewport.
     pub fn covers_viewport(map_view: &MapView) -> bool {
-        //TODO Add a little safety margin since the rendered globe is not a perfect sphere and its
-        // screen area is underestimated by the tesselation.
         let sphere_diameter = 2.0f64.powf(map_view.zoom) *
             (f64::consts::FRAC_1_PI * f64::from(map_view.tile_size));
 
-        map_view.width.hypot(map_view.height) < sphere_diameter
+        // Add a little safety margin (the constant factor) since the rendered globe is not a
+        // perfect sphere and its screen area is underestimated by the tesselation.
+        map_view.width.hypot(map_view.height) < sphere_diameter * 0.9
     }
 
     /// Returns the tile zoom value that is used for rendering with the current zoom.