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

src/core/SkDistanceFieldGen.cpp

@@ -107,6 +107,9 @@ static void init_glyph_data(DFData* data, unsigned char* edges, const unsigned c
 // computes the distance to an edge given an edge normal vector and a pixel's alpha value
 // assumes that direction has been pre-normalized
 static float edge_distance(const SkPoint& direction, float alpha) {
+#if 1    // formula (1)
+    return 0.5f - alpha;
+#else    // formula (4)
     float dx = direction.fX;
     float dy = direction.fY;
     float distance;
@@ -143,6 +146,7 @@ static float edge_distance(const SkPoint& direction, float alpha) {
     }
 
     return distance;
+#endif
 }
 
 static void init_distances(DFData* data, unsigned char* edges, int width, int height) {