This ensures that the decoration (as well as other KWin code) gets a logical size that fits
perfectly on the pixel grid. In other words, multiplying it with the scale factor results in
an integer, so there are no gaps or overlaps when using the floating point value in geometry
calculations or painting code.

For example, before, if you had a screen with 200% scale, and a window on it that's 49u wide
with 1px wide decoration borders, the border size in integer logical units would be
    round(1px / 200%) = 1u
and the resulting size
    1u + 49u + 1u = 51u => 51 * 200% = 102px
but the actual pixel size is
    1px + 49u * 200% + 1px = 100px

with this change, the floating point numbers are taken into account, the decoration border is
    1px / 200% = 0.5u
wide and thus the logical window size is
    0.5u + 49u + 0.5u = 50u => 100px
which matches the actual pixel size perfectly.

Similar issues were observable with the window size at non-integer scale factors, which also
get fixed by this commit.

Co-authored-by: Vlad Zahorodnii <vlad.zahorodnii@kde.org>

BUG: 483338
BUG: 452438
BUG: 492052