Bumps [gfx-backend-metal](https://github.com/gfx-rs/gfx) from 0.5.4 to 0.5.6.
- [Release notes](https://github.com/gfx-rs/gfx/releases)
- [Changelog](https://github.com/gfx-rs/gfx/blob/master/CHANGELOG.md)
- [Commits](https://github.com/gfx-rs/gfx/commits)

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