90 degrees image mirroring and rotation

**Is your feature request related to a problem? Please describe.**

I have a usecase where I have to deal with images that are rotated or mirrored.

**Describe the solution you'd like**

I want to be able to mirror images horizontally and vertically and rotate images by 90 degree angles.

**Describe alternatives you've considered**

egui supports rotation of images but this disables rounded corners. Mirroring and rotation by 90 degree angles are special cases where everything stays in a rectangle shape and should therefore not lead to a loss in functionality.

The issue names no files or tests. Start by tracing egui's existing image rotation path and how rounded corners are applied; done means horizontal and vertical mirroring plus 90-degree rotations work without losing rounded corners.

Written by the indexing model from the issue text.