LOD should be rounded to int

The queries should round, rather than truncate, LODs to int.

Bug 20724899

Change-Id: I86ddc724545abb80687eb5132cdf8ddd410ee2d1
Reviewed-on: https://swiftshader-review.googlesource.com/3638
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>