Fixed a lot of bugs with render surfaces and bitmaps.

samples/o3d-webgl/bitmap.js

Index: samples/o3d-webgl/bitmap.js
===================================================================
--- samples/o3d-webgl/bitmap.js	(revision 41482)
+++ samples/o3d-webgl/bitmap.js	(working copy)
@@ -74,6 +74,24 @@
 
 
 /**
+ * The scratch canvas object.
+ * @private
+ */
+o3d.Bitmap.scratch_canvas_ = null;
+
+
+/**
+ * Gets a canvas to use for scratch work.
+ * @private
+ */
+o3d.Bitmap.getScratchCanvas_ = function() {
+  if (!o3d.Bitmap.scratch_canvas_)
+    o3d.Bitmap.scratch_canvas_ = document.createElement('CANVAS');
+  return o3d.Bitmap.scratch_canvas_;
+}
+
+
+/**
  * In webgl the bitmap object is represented by an offscreen canvas.
  * @type {Canvas}
  * @private
@@ -89,6 +107,7 @@
 };
 
 
+
 /**
  * Generates mip maps from the source level to lower levels.
  *