Do not clip line width when scaling dashes

The algorithm to scale dashes was changed to no longer clip the

scaling factor: the dash patterns now continue to shrink at thin line widths.

If the line width is smaller than the effective pixel size, this may result in

dashed lines turning into solid gray-ish lines. This also required slightly

tweaking the default patterns for '--', ':', and '.-' so that with the default

line width the final patterns would not change.

There is no way to restore the old behavior.