TinyEXR-private Zstandard subset

tinyexr_zstd.c is generated from the upstream zstd source tree at /home/syoyo/work/zstd using build/single_file_libs/combine.py, with only the modules needed for one-shot compression and decompression.

TinyEXR selects the BSD license option offered by upstream zstd. The copied license text is in LICENSE; GPL is not selected.

The generated source namespaces upstream global symbols with tinyexr_zstd_ to avoid collisions when an application links TinyEXR and libzstd together.