sdf: use linear edge distance approximation for all gradient directions

Image diff tools tell me there are diffs, but I sure can't see them.
This drops edge_distance() off the chart as far as CPU cost goes.

BUG=skia:4729
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1545893003

Review URL: https://codereview.chromium.org/1545893003
1 file changed