by existing WebGL conformance tests. 5x speedup observed for reading back a
4096x4096x4 texture on a NVIDIA GeForce 9600M GT (from 1000ms to 200ms).

Review URL: https://codereview.appspot.com/7466044
Author: Evan Wallace

git-svn-id: https://angleproject.googlecode.com/svn/trunk@1999 736b8ea6-26fd-11df-bfd4-992fa37f6226