BUG: rot only works if there is a $vp slot

[MHenderson]

The current behaviour is to not rotate if $vp is NULL. But even that seems wrong. We shuold create the viewport and then rotate. Also testing for $vp being NULL assumes that $vp is defined, doesn't it?

[MHenderson]

I think the answer is just to create the slot if it doesn't exist.

[MHenderson]

Write a test.