[BUG] D2D renderer does not draw purely horizontal/vertical paths

[yfede]

The current isEmpty() returns whether the area of the rectangle is 0 (i.e. at least one of the dimensions is 0).

I think this case shows that it would be helpful to also have a method that returns whether both the dimensions are 0

I proposed such addition a while ago, feel free to vote it:

Companion for Rectangle::isEmpty()

I found myself in the peculiar condition of having to distinguish when a Rectangle has zero area (i.e. one of its dimensions is zero) vs. when the Rectangle has both dimensions equal to zero, not just one. The Rectangle::isEmpty() method covers in the first case (while I was expecting it to be more fitting for the latter, but anyway…), and there seem to be no method for the latter. I know this can be easily added with a free function, and that’s what I’m doing with: template <typename ValueTy…