Refine order of previous/next screens in Workspace::findOutput

A screen should be considered further top, and therefore more previous, only it is strictly above the other with no vertical overlap, and not if only the vertical center is higher while they are horizontally on a line. Otherwise, on a left to right setup with screens of different resolutions aligned at the edges (and thus different vertical centesr), the order will be unintuitive.

BUG: 467996
FIXED-IN: 5.27


(cherry picked from commit 9a3da098)