Fixed warnings related to type conversions leading to potential loss of precision.

BUG=18368388

Change-Id: I71a7941df4bcf991f04818060780d4d395e335a9
Reviewed-on: https://swiftshader-review.googlesource.com/1393Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>