Fixed loading UInt textures

Fixed a copy-paste error where only half
the required number of bytes were loaded.

Change-Id: I43ea2e1b5368137e19fb6bb20c03b5c9ca10ba3b
Reviewed-on: https://swiftshader-review.googlesource.com/5141
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>