For one, it seems this was the intention already since commit
85c70f37 (CWS-TOOLING: integrate CWS
aw063, 2009-03-04): "suppress AntiAliasing for pure horizontal or
vertical lines".

For another, this fixes the TileCacheTests::testTileWireIDHandling()
testcase in online.git, which assumes that the indicators at the corners
of the Writer body frame (paragraph marks hidden / default case) can be
painted multiple times, producing pixel-by-pixel matching results. But
in reality AA breaks that assumption, and we know these indicators are
never diagonal lines.

Change-Id: Ib74f823165799991296b64cee58ec106dbdcedcf
Reviewed-on: https://gerrit.libreoffice.org/72000Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins