add Animation constructor with just frame length parameter

src/Animation.hpp

@@ -32,10 +32,10 @@ public:
     Animation();
 
     /*!
-     * @deprecated It is preferable to use the default constructor, provide a frame length using Animation::frame_length(float), and
-     * omit the callback function. This is because storing the callback function in this object can create copying issues when
-     * std::bind is used to create the callback. Instead, check the return value of Animation::update(float) to determine whether or
-     * not to call the desired callback externally.
+     * @deprecated It is preferable to use Animation::Animation() or Animation::Animation(float) and omit the callback function,
+     * which will eventually be removed from this object. This is because storing the callback function can create copying issues
+     * when std::bind is used to create the callback and the bound object is destroyed before the callback is called. Instead,
+     * check the return value of Animation::update(float) to determine whether or not to call the desired callback externally.
      * 
      * Create an Animation object by supplying a function and interval at which the function should run. Run Animation::update(float)
      * regularly with an updated timestamp from Game::update(float), and the function will be launched automatically at the given
@@ -49,6 +49,17 @@ public:
         timer.off();
     }
 
+    /*!
+     * Create an Animation object with a specific amount of time in seconds between each frame. Animation::update(float) will only
+     * return true when it is time to display another frame.
+     *
+     * @param frame_length   seconds between each frame
+     */
+    Animation(float frame_length) : _frame_length(frame_length)
+    {
+        timer.off();
+    }
+
     /*!
      * Set the duration in seconds of each frame of the animation.
      *