fdo#75260: Center both primary and secondary lines wrt thier respective axes.

The closed borders connect much better this way.  Now all double lines look
at least half-way decent.

Also, we shouldn't be doing any sort of pixel correction in the common code
that gets used both raster and vector drawing modes.  In case we need to do
some pixel correction, we should do it in the pixel processor code.

Change-Id: I78e245c10752d0f4790a9ac664a14ce06571434e