We were relying on the fact that we wouldn't paint anything on top
of the black background in the region of a black rect. However
when fractional scaling is used the repaint region might get
expanded to nearby pixels by scale_output_damage(). As a result
the neighbour scene nodes might leak into the skipped black rect's
region.

Avoid this by using this optimization for bottom-most black rects
only when fractional scaling is used.

References: https://github.com/swaywm/sway/issues/8233