Bump src/deps/Imath from 3ad5d4d to 6add873

Bumps [src/deps/Imath](https://github.com/AcademySoftwareFoundation/Imath) from `3ad5d4d` to `6add873`.
- [Release notes](https://github.com/AcademySoftwareFoundation/Imath/releases)
- [Commits](AcademySoftwareFoundation/Imath@3ad5d4d...6add873)

---
updated-dependencies:
- dependency-name: src/deps/Imath
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]