Bump third_party/imgui/repo from 3c435c0 to ccb6646

Bumps [third_party/imgui/repo](https://github.com/ocornut/imgui) from `3c435c0` to `ccb6646`.
- [Release notes](https://github.com/ocornut/imgui/releases)
- [Commits](ocornut/imgui@3c435c0...ccb6646)

---
updated-dependencies:
- dependency-name: third_party/imgui/repo
  dependency-type: direct:production
...

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