Output rotation

Created by: ascent12

I was looking into output rotation, but due to how trivial it is to implement with OpenGL, I don't really think there is a lot of value wlroots could add by "supporting" this directly; the library user can do it themselves. Maybe there is a flag somewhere that we needs to be set, just so we know that an output is rotated, but until I know that we actually need that, I won't bother adding it.

On that note, I wrote another example program which supports output rotation, and is configurable with its command line options. It also renders something different, so that you can actually tell if the rotation is working.