Move glClear call out of the critical path

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));
+
+
             //TODO increase atlas size earlier to avoid excessive copying to the GPU
             //TODO increase max tile cache size?
             increase_atlas_size = {

src/map_view_gl.rs

             center: self.map_view.center,
         });
 
-        self.cx.clear_color((0.2, 0.2, 0.2, 1.0));
-
         let visible_tiles = self.map_view.visible_tiles(true);
         let mut remainder = visible_tiles.as_slice();
         let mut num_draws = 0;