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