`QGuiApplication::primaryScreen()->grabWindow(...)` only includes pixels
below and to the right of the top left corner of the primary screen (at
least in Linux). Consider a screen arrangement like:
+-------------+
+---------++---------+| |
+ || || |
+ 3 || PRIMARY || 2 |
+---------++---------++-------------+
With this screen arrangement, the top of screen 2 and all of screen 3
would be left out of the screenshot.
Combining the results of calling `grabWindow(...)` on each `QScreen`
resolves the issue.
