effects/overview: Correct size of active virtual desktop highlight outline

The focus outline that highlights the active virtual desktop in the Overview effect, as
well as the half-transparent outlines that are around the other virtual desktops, cover
2 pixels on every side of every virtual desktop thumbnail.

This is because an invisible rectangle is used that has the same size as the thumbnail
and has a colored border, but borders are always drawn on the inside of QML rectangles,
so content in the thumbnail is covered up. Therefore, the width of the rectangle has to
be slightly adjusted to account for the thickness of the border.

BUG: 450963
FIXED-IN: 5.27