Bumps [imageproc](https://github.com/image-rs/imageproc) from 0.20.0 to 0.21.0.
- [Release notes](https://github.com/image-rs/imageproc/releases)
- [Changelog](https://github.com/image-rs/imageproc/blob/master/CHANGELOG.md)
- [Commits](https://github.com/image-rs/imageproc/commits)

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