[gui] angled labels didn't calculate the appropriate dirty region. 

This is quite ugly, but fixes a bug with DR mode 1.

Ideally the <angle> facility would be removed post-Gotham.  I don't believe there is any need for it as the rotatez animation should be able to accomplish the same thing, whereby this would automatically be taken care of.

Fixes #14242.

@ronie: Any idea if `<angle>` goes (post-Gotham) whether it would be missed?