Added explicit casts between geometry types.

This is probably preferable to into<C>().

example/fireworks.cxx

@@ -248,13 +248,13 @@ void Fireworks::draw_fireworks(Sprite_set& sprites) const
         switch (firework.stage) {
             case Firework::Stage::mortar:
                 sprites.add_sprite(view.mortar,
-                                   firework.mortar.position.into<int>());
+                                   Position(firework.mortar.position));
                 break;
 
             case Firework::Stage::stars:
                 for (Projectile const& star : firework.stars) {
                     sprites.add_sprite(view.stars[firework.star_color],
-                                       star.position.into<int>());
+                                       Position(star.position));
                 }
                 break;
 
@@ -303,8 +303,8 @@ void Fireworks::on_key(Key key)
         is_paused = !is_paused;
     } else if (key == Key::code(' ') && !is_paused) {
         auto dims = get_window().get_dimensions();
-        auto initial_position = Position{dims.width / 2, dims.height};
-        model.add_random(get_random(), initial_position.into<double>());
+        auto pos0 = Projectile::Position(dims.width / 2., dims.height);
+        model.add_random(get_random(), pos0);
     }
 }
 
@@ -321,6 +321,7 @@ void Fireworks::on_frame(double dt)
 
 void Fireworks::on_mouse_up(Mouse_button, Position position)
 {
-    if (!is_paused)
-        model.add_random(get_random(), position.into<double>());
+    if (is_paused) return;
+
+    model.add_random(get_random(), Projectile::Position(position));
 }

include/ge211_geometry.hxx

@@ -41,6 +41,15 @@ struct Basic_dimensions
     {
         return {U(width), U(height)};
     }
+
+    /// Alias for `into<U>()` to support casting between coordinate
+    /// types.
+    template <class U>
+    explicit operator Basic_dimensions<U>() const
+    NOEXCEPT_(noexcept(into<U>()))
+    {
+        return into<U>();
+    }
 };
 
 /// Type alias for the most common use of Basic_dimensions, which is with
@@ -254,6 +263,15 @@ struct Basic_position
         return {U(x), U(y)};
     }
 
+    /// Alias for `into<U>()` to support casting between coordinate
+    /// types.
+    template <class U>
+    explicit operator Basic_position<U>() const
+    NOEXCEPT_(noexcept(into<U>()))
+    {
+        return into<U>();
+    }
+
     /// @}
 
     /// \name Shifting member functions
@@ -423,6 +441,15 @@ struct Basic_rectangle
         return {U(x), U(y), U(width), U(height)};
     }
 
+    /// Alias for `into<U>()` to support casting between coordinate
+    /// types.
+    template <class U>
+    explicit operator Basic_rectangle<U>() const
+    NOEXCEPT_(noexcept(into<U>()))
+    {
+        return into<U>();
+    }
+
     /// Creates a Basic_rectangle given the position of its top left vertex
     /// and its dimensions.
     static Basic_rectangle from_top_left(Position tl, Dimensions dims)