Make adjacent frame borders overlap

Currently, musca gives each frame its own four borders. Between frames, the borders could be made to overlap. With a border width setting of 1 pixel, and 3 frames side by side, that means the border between two frames is not efficiently reused to highlight the focused frame because 1 pixel band of its width simply has the focus colour whereas the other is probably of a lesser visible colour, instead of having a 1 pixel border between frames that can be highlighted by whatever frame it shares that happens to be focused. Of course this also leads to a loss of workspace that windows could otherwise have used instead. Needless to say the problem gets worse with border_width>1.