Added proper constness to Context getters.

Change-Id: I0495cbe7ab0c9f50d4d9aa3d969507f117539bd8
Reviewed-on: https://swiftshader-review.googlesource.com/2983Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>