Added utility function for OpenGL extension strings

The new glGetStringi() function needed to be unified
with glGetString(), so I made a utility function
to make future modifications easy.

Change-Id: I6619d9f99b10d9fc43bd8783a1d50274e2b37a7b
Reviewed-on: https://swiftshader-review.googlesource.com/2797
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>