Fix rounding issues with texture generation shader

Fix rounding issues with texture generation shader

VK-GL-CTS Issue: 3250

Affects:

dEQP-VK.graphicsfuzz.cov-min-nested-loop-same-value-for-variables
dEQP-VK.graphicsfuzz.cov-sample-texture-hundred-iterations
dEQP-VK.graphicsfuzz.cov-sampler-as-function-argument
dEQP-VK.graphicsfuzz.cov-texel-double-negation
dEQP-VK.graphicsfuzz.cov-variable-copy-in-function-tex-sample
dEQP-VK.graphicsfuzz.spv-stable-sampler-loop-extra-instructions
dEQP-VK.graphicsfuzz.spv-stable-sampler-polar-simple-O-access-chain

Components: Vulkan
Change-Id: I1758bd50da844b2835c858ee5d78af4e7c4bd69a