I'm looking for a simple function to rotate a sprite (or just one sprite tile) by 90 degrees to reduce sprite-sheet usage. We can already easily flip a sprite, so surely a 90-degree rotation isn't that far off, given we can already do it with the R key in the sprite editor.

I've spent a lot of time searching and seen lots of examples of free rotation to any angle you want. They all look very nice, but they use a lot of code and CPU time. All I want is the ability to rotate by 90 degrees, which I imagine would be a lot simpler and take a lot less CPU time and code, since there's no angle calculations or trigonometry, just a simple rows/columns operation.

Before I try to get my head around it, I was wondering if anyone else already has an elegant solution?