add rotation view

[behzadrocket]

One of the nice to have view modes is to be able to rotate display of a sequence by 90,180,270

[toaaot]

yes - rotating (and mirroring) the image would be very useful for me. Should be controllable by file name, too.

[ChristianFeldmann]

Ah but that is just for the rendering, right? But it may be a bit complicated with the other features.
I files a feature request issue for this: #471