Fixed an error when native canvas isn't available.

The recent changes to add retina support included a call to getContext
made before excanvas/flashcanvas had a chance to register their hooks;
fixed by moving that code up a few lines.

jquery.flot.js

@@ -735,6 +735,9 @@
             var c = document.createElement('canvas');
             c.className = cls;
 
+            if (!c.getContext) // excanvas hack
+                c = window.G_vmlCanvasManager.initElement(c);
+
             var cctx = c.getContext("2d");            
 
             // Increase the canvas density based on the display's pixel ratio; basically
@@ -754,9 +757,6 @@
 
             $(c).appendTo(placeholder);
 
-            if (!c.getContext) // excanvas hack
-                c = window.G_vmlCanvasManager.initElement(c);
-
             // Save the context so we can reset in case we get replotted
 
             cctx.save();