zink: lower stipple + smooth

When computing line smoothing, we can also do something similar to compute the line stippling. This can be useful for some drivers, who can't easily split the lines before rasterizing them.

This does lead to slightly inaccurate stippling, because the line-smoothing extends the line-length by a small amount. That leads to the line-stippling pattern being over-stretched over the line-segment by a fraction of a pixel in length. For short lines, this can be quite a

Without this, we use aliased stippling even when using smooth lines.