add mobile safari workarounds and query parameters to reduce memory i…

…n ThreeStage, phetsims/density-buoyancy-common#316

Signed-off-by: Michael Kauzmann <michael.kauzmann@colorado.edu>

js/initialize-globals.js

@@ -456,13 +456,6 @@
      */
     postMessageOnReady: { type: 'flag' },
 
-    /**
-     * Controls whether the preserveDrawingBuffer:true is set on WebGL Canvases. This allows canvas.toDataURL() to work
-     * (used for certain methods that require screenshot generation using foreign object rasterization, etc.).
-     * Generally reduces WebGL performance, so it should not always be on (thus the query parameter).
-     */
-    preserveDrawingBuffer: { type: 'flag' },
-
     /**
      * If true, the full screen button won't be shown in the phet menu
      */