* Reevaluate all featuers stroke widths for shadow, casing, stroke
* Shadow width is important to make sure all features are easily clickable
* Casing width is important to see oneway arrows
* Make sure all line styles have normal and low-zoom variants
* Show directional arrows for all waterway types
The interpolator now just runs a few discrete steps, avoiding most of the
repainting that was causing high CPU and noisy laptop fans. And it still
looks pretty good.
(closes#2911)