Implemented glGetFragDataLocation. There's very little coverage for
this function in dEQP, but the one test that uses it passes.

Change-Id: I6cfc93d7eaad54f23c922e45dafee9cba3401c83
Note: Chromium's ES3 path requires this function to be implemented.
Reviewed-on: https://swiftshader-review.googlesource.com/11728Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>