draw: fix line stippling with aa lines

draw: fix line stippling with aa lines

In contrast to non-aa, where stippling is based on either dx or dy
(depending on if it's a x or y major line), stippling is based on
actual distance with smooth lines, so adjust for this.

(It looks like there's some minor artifacts with mesa demos
line-sample and stippling, it looks like the line endpoints
aren't quite right with aa + stippling - maybe due to the
integer math in the stipple stage, but I can't quite pinpoint it.)

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>