BUG: 361053
When taking a screenshot with the rectangle selection tool in spectacle,
the overlaid help-text is put in the center of the entire desktop,
instead of the center of a single display. This can cause readability
issues when it is placed on the bezel of two physical displays.
This patch horizontally centers the help-text overlay on the primary display.
This affects both the "Click anywhere to start" dialogue and the hotkey help-text.