message: Update RELEASE_NOTES for 1.9.1
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
  +## v1.9.1
  +
  +- Improvements to SSAO quality
  +- Fix unoptimized shader crashes with certain OpenGL drivers
  +- Add float versions of math constants to libmath
  +- filament-utils: fix, `CoroutineScope` job should be canceled before destroy
  ## v1.9.0
  