Draco 1.5.6 release

Version 1.5.6 release:

• Using the versioned www.gstatic.com WASM and Javascript decoders continues
  to be recommended. To use v1.5.6, use this URL:
  • https://www.gstatic.com/draco/versioned/decoders/1.5.6/*
• The CMake flag DRACO_DEBUG_MSVC_WARNINGS has been replaced with
  DRACO_DEBUG_COMPILER_WARNINGS, and the behavior has changed. It is now a
  boolean flag defined in draco_options.cmake.
• Bug fixes.
• Security fixes.