message: Update RELEASE_NOTES for 1.9.3
diff: This file contains one line summaries of commits that are worthy of mentioning in release notes.
  A new header is inserted each time a *tag* is created.
  -## Next release (v1.9.4)
  +## Next release (v1.9.5)
  +
  +## v1.9.4
  - New `ShadowOptions` control to render Variance Shadow Maps (VSM) with MSAA (experimental).
  ## v1.9.3
  +- engine: Added new APIs to enable/disable screen space refraction
  +- engine: Fix, flip the shading normal when det < 0.
  +- gltfio: Fix animation by clamping the per-channel interpolant.
  +- gltfio: add async cancellation API
  +- gltfio: Fix "uniform not found" errors.
  +- gltfio: Disable clear coat layer IOR change in glTF files (#3104)
  +- Vulkan: fix final image barrier used for swap chain.
  +- matdbg: Various improvements
  +- JavaScript bindings: fix TextureUsage bitmask.
  +- cmgen / mipgen: add opt-in for ASTC / ETC support.
  +
  ## v1.9.2
  - Fixes / improvements for contact shadows, fog, and DOF
  