Added immutable levels info to Texture and added the query.

Change-Id: I900a3c433dcc63b8113b1f623da3518c00680e16
Reviewed-on: https://swiftshader-review.googlesource.com/4751Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>