Implement half-precision float quantization
Implement half-precision float quantization

quantizeHalf handles float->half conversion, correctly preserving
+-Inf/NaN, generating +-Inf on overflow, and rounding to nearest value.

Denormals are flushed to zero, both to simplify the conversion and to
eliminate the performance cost of handling denormals on target hardware
(in case some GPUs don't support full rate fp16 denormals).

The results were verified by comparing them against ISPC float->half on
all 2^32 floats; the only source of difference is flush-to-zero for
denormals.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer