UI: Center preview spacing lines

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Currently, the spacing lines begin in the middle of the box and then are
their width wide towards one direction. This means that the larger they
are, the more off-center their middle is.
This commit changes them to start at half their width left/top of the
center and end half the width to the right/bottom of the center, putting
the middle in the center.

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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f