Thick lines are drawn by gdImageFilledRectangle(), which iterates over
the x ordinate first (Z order) to apply the style pattern. While this works
fine for vertical and diagonal lines, it obviously fails for horizontal
lines, which have to be iterated over in N order.
To fix this bug, we introduce the helpers gdImageFilled(H|V)Rectangle(),
which may be reused for other purposes as well.
This is basically the same fix as libgd/libgd/c2b91dbc.
