Draco 1.5.7 release

Version 1.5.7 release:

• Using the versioned www.gstatic.com WASM and Javascript decoders continues
  to be recommended. To use v1.5.7, use this URL:
  • https://www.gstatic.com/draco/versioned/decoders/1.5.7/*
• Added support for normalized attributes to Emscripten encoder API.
• Bug fixes.
• Security fixes.