glGetString wasn't returning es3 specific extensions.

Change-Id: Ief89eb448b3e821bfb7919ece97c03976569060c
Reviewed-on: https://swiftshader-review.googlesource.com/10954Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>