fdo#73487: Write unit tests for this.

Also remove the files used for the old tests.

Change-Id: I4fbf6acce205d15a35780524120e351e01dc6642
Draw horizontal solid border lines directly in the pixel processor.

This makes slightly skinnier solid lines which look better on screen.

Change-Id: Ia7764be4a53d1dd6bb60ecb3ba5c8966403e4e6c