Adding support for OES_standard_derivatives extension. This is not the complete implementation. Sending it to get feedback on the API. Is it OK to add extension support into TBuiltInResource? I could create a new struct for extensions but that would lead to API change.
BUG=25
Review URL: http://codereview.appspot.com/1953047

git-svn-id: https://angleproject.googlecode.com/svn/trunk@402 736b8ea6-26fd-11df-bfd4-992fa37f6226