Floating point negative values were first converted to half values,
followed with a conversion to 11 bit or 10 bit mini floats by
chopping the mantissa and removing the sign bit. Negative values
were converted to the same values as positive values since removing
the sign bit ended up doing the equivalent of an Abs() call.

Clamped the value to 0 before the conversion to solve the issue.

Bug: b/146223877 b/147900455
Change-Id: I97decae66dc57a68f175b06902eb6725fc7d5794
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/42548
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>