Add mirror (vertical flip) option

[mryndzionek]

• [x] Check that you've added documentation to any new methods
• [x] Rebase from master if you're not already up to date
• [ ] Add or modify an example if there are changes to the public API
• [x] Add a CHANGELOG.md entry in the Unreleased section under the appropriate heading (Added, Fixed, Changed, etc)
• [x] Run rustfmt on the project with cargo fmt --all - CI will not pass without this step
• [x] Check that your branch is up to date with master and that CI is passing once the PR is opened

PR description

Added a function for mirroring/flipping the display. Not that elegant, but seems to be working (tested on one display so far). Let me know if this is acceptable.

[jamwaffles]

Thanks for the PR! I'll give this a test on my sample displays when I've got a spare minute :)