Glitch in strut size calculation

There's some glitch in the struts rect calculation: width/height is always 1px too small.

Cause: the wm hint contains the start and end position (instead of start + size). Thus, just taking end-start for the size isn't enough: we have to add 1, but only if the end-start > 0.

The spec is a bit imprecise here: we don't really know to detect whether some strut is present or not, if it's only 1px wide/high - just assume that end-start==0 means not present. (the current implementation causes that implicitly: we're ending up w/ zero width/height rect, which is treated empty by gdk_rectangle_intersect).

Fix: add 1 to the width/height, but only if it's already greater than 1.

Yet unclear: what happens if end < start ?