Add extension to create software device. Add an extension so that users can receive a device that renders in software. Currently this is done by loading a swiftshader dll. BUG= TEST= Review URL: http://codereview.appspot.com/4631093 git-svn-id: https://angleproject.googlecode.com/svn/trunk@705 736b8ea6-26fd-11df-bfd4-992fa37f6226