I am looking for a shader which supports rotating the screen output 90 degrees. Any help appreciated.
That would be a very simple shader. If you search around for how basic post process shader code looks and how it works, I’m sure it wouldn’t take too much effort to get the results what you want.
However, unless your screen is an exact square, the result will be stretched and you will lose quality. It would look better and be much easier to implement if you simply rotate your camera along the Z axis, rotating the image without any post processing.