Scaling differently sized WindowPreviews by a constant factor will
result in smaller windows getting enlarged by a smaller amount of pixels
than larger windows (1000*1.02=1020 but 100*1.02=102, one will grow by
20 pixels and the other one by 2), this can look a bit weird because
smaller windows don't scale up as much as larger windows.
So introduce a constant extra size to use when scaling windows up, we
set only the half size there because we want to ensure that the size
added on both sides is not fractional and we remain aligned to the pixel
grid.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1692>
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