Update Alligned Rect Shader to match geometry version

src/gpu/GrAARectRenderer.cpp

Index: src/gpu/GrAARectRenderer.cpp
diff --git a/src/gpu/GrAARectRenderer.cpp b/src/gpu/GrAARectRenderer.cpp
index f28d91456308143206a0f77310df60cd8a24f263..142f6a8232d1aef1801acf09b0897bd0cc2d8d38 100644
--- a/src/gpu/GrAARectRenderer.cpp
+++ b/src/gpu/GrAARectRenderer.cpp
@@ -59,23 +59,28 @@ public:
                 builder->getEffectAttributeName(drawEffect.getVertexAttribIndices()[0]);
             builder->vsCodeAppendf("\t%s = %s;\n", vsRectName, attr0Name->c_str());
 
-            // TODO: compute these scale factors in the VS
-            // These scale factors adjust the coverage for < 1 pixel wide/high rects
-            builder->fsCodeAppendf("\tfloat wScale = max(1.0, 2.0/(0.5+%s.z));\n",
-                                   fsRectName);
-            builder->fsCodeAppendf("\tfloat hScale = max(1.0, 2.0/(0.5+%s.w));\n",
-                                   fsRectName);
+            // TODO: compute all these offsets, spans, and scales in the VS
+            builder->fsCodeAppendf("\tfloat insetW = min(1.0, %s.z) - 0.5;\n", fsRectName);
+            builder->fsCodeAppendf("\tfloat insetH = min(1.0, %s.w) - 0.5;\n", fsRectName);
+            builder->fsCodeAppend("\tfloat outset = 0.5;\n");
+            // For rects > 1 pixel wide and tall the span's are noops (i.e., 1.0). For rects
+            // < 1 pixel wide or tall they serve to normalize the < 1 ramp to a 0 .. 1 range.
+            builder->fsCodeAppend("\tfloat spanW = insetW + outset;\n");
+            builder->fsCodeAppend("\tfloat spanH = insetH + outset;\n");
+            // For rects < 1 pixel wide or tall, these scale factors are used to cap the maximum
+            // value of coverage that is used. In other words it is the coverage that is
+            // used in the interior of the rect after the ramp.
+            builder->fsCodeAppend("\tfloat scaleW = min(1.0, 2.0*insetW/spanW);\n");
+            builder->fsCodeAppend("\tfloat scaleH = min(1.0, 2.0*insetH/spanH);\n");
 
             // Compute the coverage for the rect's width
-            builder->fsCodeAppendf("\tfloat coverage = clamp(wScale*(%s.z-abs(%s.x)), 0.0, 1.0);\n",
-                                   fsRectName,
-                                   fsRectName);
-
+            builder->fsCodeAppendf(
+                "\tfloat coverage = scaleW*clamp((%s.z-abs(%s.x))/spanW, 0.0, 1.0);\n", fsRectName,
+                fsRectName);
             // Compute the coverage for the rect's height and merge with the width
             builder->fsCodeAppendf(
-                    "\tcoverage = min(coverage, clamp(hScale*(%s.w-abs(%s.y)), 0.0, 1.0));\n",
-                    fsRectName,
-                    fsRectName);
+                "\tcoverage = coverage*scaleH*clamp((%s.w-abs(%s.y))/spanH, 0.0, 1.0);\n",
+                fsRectName, fsRectName);
 
             SkString modulate;
             GrGLSLModulatef<4>(&modulate, inputColor, "coverage");