Fix rounding when casting to integer

Fix rounding when casting to integer

Normalized values that were sampled from textures and have been scaled
by their maximum magnitude will be close to their unnormalized integer
value but may be slightly less. Subsequent casting to integer (with
rounding towards zero) may therefore result in a value one-off from the
original.

Use round-to-nearest to ensure the correct integer value is produced.

Fixes: #151
VK-GL-CTS public issue: 151
VK-GL-CTS issue: 1853
Affects: dEQP-VK.memory.pipeline_barrier.all.*
Components: Vulkan
Change-Id: I435d41145c83957db15c74ea394e89faf142c2a3