With some resolutions (such as 4096x2160) we may compute duplicated
scale factors because we used a too wide threshold to check for an
applicable value.
In fact, while when we're at the first and last values it's fine to
search applicable values up to SCALE_FACTORS_STEP, on intermediate ones
we should stop in the middle of it, or we're end up overlapping the
previous scaling value domain.
In the said example in fact we were returning 2.666667 both when
looking to a scaling value close to 2.75 and 3.00 as the upper bound of
2.75 (3.0) was overlapping with the lower bound of 3.0 (2.75).
With the current code, the lower and upper bounds will be instead 2.875.
Adapt test to this, and this allows to also ensure that we're always
returning a sorted and unique list of scales (which is useful as also
g-c-c can ensure that this is true).
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1878>
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