Let horizontal-line be a straight-cut line rather than having rounded edges (issue 8685043)

[thomasmorley65]

Can't review C++ -code, though, from description, I do not understand
the reason for it.

Well, ofcourse it will merge better with barlines, but can you give an
example where the problem isn't solvable with at most medium effort?
Ah, wait, I just found
http://lsr.dsi.unimi.it/LSR/Search?q=color
Regarding it with very high zoom shows a greater disadvantage of
draw-line.
Nevertheless, I think it's more a problem of said snippet, worth fixing.

In general, I'd prefer rounded boxes, but with a little blot-diameter.
Though, I might change my mind once I understand the reason behind the
patch.

https://codereview.appspot.com/8685043/