title: Purging a scene causes global display objects to be removed from memory
body: <p>I have a problem whereby when I call storyboard.purgeScene(), my global display objects seem to be removed. When I say 'global', they I actually store them in a user-defined storyboard table, to avoid using actual global variables:</p>
  
  <p>My image database is stored in external file toy_database.lua:</p>
  
  <pre><code>local storyboard = require "storyboard";
  
      storyboard.globalVars.toys = {
              ball1 = {
                      src = 'img/stock-vector-soccer-ball-trimmed.png',
              }
      }
  </code></pre>
  
  <p><br/>main.lua:</p>
  
  <pre><code>local storyboard = require "storyboard";
  
  storyboard.globalVars = {};
  
  require "toy_database_toybox";
  </code></pre>
  
  <p><br/>I load some images in my main.lua:</p>
  
  <pre><code>storyboard.globalVars.toys["ball1"].imgPtr = display.newImageRect(storyboard.globalVars.toys["ball1"].src, w, h);
  </code></pre>
  
  <p><br/>Then I go to another scene:</p>
  
  <pre><code>storyboard.gotoScene("mainFloorScene");
  </code></pre>
  
  <p><br/>where everything is fine, the display objects are accessable. I purge that scene and goto another scene:</p>
  
  <pre><code>storyboard.gotoScene( "openBoxScene" )
  [in didExitScene()]: storyboard.purgeScene( "mainFloorScene" )
  </code></pre>
  
  <p><br/>Everything is fine here too. Finally I go back to the first scene, purging the current scene on the way:</p>
  
  <pre><code>storyboard.gotoScene( "mainFloorScene", "fade", 400 );
  [in didExitScene()]: storyboard.purgeScene( "openBoxScene" )
  </code></pre>
  
  <p><br/>Back in the first scene the image objects have been removed. If I comment out the line:</p>
  
  <pre><code>storyboard.purgeScene( "openBoxScene" )
  </code></pre>
  
  <p><br/>it works fine, so I have a workaround but I'd like this behaviour explained. I expected a scene purge to only remove display objects local to that scene.</p>
  
  <p>What's going on?</p>