sdf: use linear edge distance approximation for all gradient directions

src/core/SkDistanceFieldGen.cpp

Index: src/core/SkDistanceFieldGen.cpp
diff --git a/src/core/SkDistanceFieldGen.cpp b/src/core/SkDistanceFieldGen.cpp
index 30354e09f83cb783c49ee018af74b3d8e236b272..c147151c3b21def3fcef267e30b972b86593152c 100755
--- a/src/core/SkDistanceFieldGen.cpp
+++ b/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) {