Implemented GL_SAMPLER_BINDING query

Sampler objects already exist, only the query was missing.

Change-Id: Iea4a65ee4af149a33b262eba4fc0d6256775d1e1
Reviewed-on: https://swiftshader-review.googlesource.com/4746
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>