Don't call glClear when map fills the whole screen

src/main.rs

             let _ = gl_window.swap_buffers();
 
             // Move glClear call out of the critical path.
-            //TODO do not call glClear when drawing fills the whole screen anyway
-            cx.clear_color((0.2, 0.2, 0.2, 1.0));
-
+            if !map.viewport_in_map() {
+                cx.clear_color((0.2, 0.2, 0.2, 1.0));
+            }
 
             //TODO increase atlas size earlier to avoid excessive copying to the GPU
             //TODO increase max tile cache size?

src/map_view.rs

         }
     }
 
+    /// Returns true if the viewport rectangle is fully inside the map.
+    pub fn viewport_in_map(&self) -> bool {
+        let scale = f64::powf(2.0, -self.zoom) / f64::from(self.tile_size);
+
+        let y_top = self.center.y + -0.5 * self.height * scale;
+        let y_bottom = self.center.y + 0.5 * self.height * scale;
+
+        y_top >= 0.0 && y_bottom <= 1.0
+    }
+
     /// Returns the screen coordinate of the top-left corner of a tile.
     pub fn tile_screen_position(&self, tile: &TileCoord) -> ScreenCoord {
         self.map_to_screen_coord(tile.map_coord_north_west())

src/map_view_gl.rs

         self.cx.set_viewport(0, 0, width, height);
     }
 
+    pub fn viewport_in_map(&self) -> bool {
+        self.map_view.viewport_in_map()
+    }
+
     pub fn increase_atlas_size(&mut self) -> Result<(), ()> {
         self.tile_atlas.double_texture_size()
     }