Return correct OpenGL ES version

Since we can create OpenGL ES 3.0 contexts locally now,
we should also be able to retrieve the correct version.

Change-Id: I9154869ddea5951946c8e23e59a00beb042fca8c
Reviewed-on: https://swiftshader-review.googlesource.com/3633
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>