Following the Vulkan 1.1 specification Section 2.9.1. "Conversion from
Normalized Fixed-Point to Floating-Point":

Note that while zero is exactly expressible in this representation, one
value (-128 in the example) is outside the representable range, and must be
clamped before use.

https://www.khronos.org/registry/vulkan/specs/1.1/html/vkspec.html#fundamentals-fixedfpconv

Bug: dawn:283
Change-Id: Id548c6d67132ac36f33bc020954c5511de624ffc
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43648
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Corentin Wallez <cwallez@google.com>