Rounding toward zero never produces infinity from a finite value: past the
f16 range the result is +-65504. We truncated the mantissa and let
packHalf2x16 round, so large values became infinite. Silent Hill's deferred
lighting packs title-menu values past 65504; its height fog then multiplies
scene color by a tiny transmittance, which keeps 65504 finite but leaves an
infinity infinite, a later pass turns it into NaN and the whole 3D menu
background renders black. Subnormal results now also keep whole 2^-24 steps.
A MoltenVK device test covers overflow, truncation, subnormals, NaN and inf.
52ba493155