By default, the Ctrl-M shortcut is not working in the VSE preview.
This PR aims to address the issue and provide a working iteration of
the feature.
In my view, it would be preferable for this operator to not only flip
the `flip_x` and `flip_y` values, but also manipulate the transform and
rotation of the strips in a way that keeps the strip in the same
location on the screen, rather than flipping them to the other side.
I decided to implement the feature with this correction functionality,
as it seems more coherent with how the mirror works in other areas of
Blender.
Pull Request: https://projects.blender.org/blender/blender/pulls/136343
12f5ce7c85