threedim: software half-float conversion in ArrayToTexture

On glibc < 2.40 there is no _Float16, so the R16F/RGBA16F case compiled to
nothing and the texture was uploaded with uninitialized bytes. Convert with
a software IEEE 754 binary16 (round-to-nearest-even) fallback instead.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014rZgzE8JjWvHDtaVUhxpLE

Author: jcelerier