applets/pager: fix window/screen size/position when using Qt scaling

On X11, the rect returned from QScreen::virtualGeometry cannot be used
as the whole screen rect. Instead, compute the whole screen rect
manually, and map all windows/screens to the non-scaled coordinate
system on X11 to make windows show in right positions in the pager applet.

BUG: 446627
FIXED-IN: 5.27.8

(cherry picked from commit 18b91601)