Allow all rotation angles

Fixes #194.

I hope my viewport calculations are correct.

I also have code which allows for pinch rotate, would you like me to create a PR for that too? If so, should I add an option for it?
Yes, I have. I didn't see any changes. The content in the navigator does not get resized properly though. But I don't think removing that code is the cause (we've got a bug for this #463).