Add small offset to t in GrLinearGradientLayout.

This works around a varying interpolation issue when a hard stop occurs
a along a row or column of pixel centers.

Bug: chromium:938592
Change-Id: I4c239b2831c3e901cbbfa43f0a60a3d7fb0fef75
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/200939
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>

src/gpu/gradients/GrLinearGradientLayout.cpp

@@ -25,8 +25,9 @@ public:
         auto gradientMatrix = _outer.gradientMatrix();
         (void)gradientMatrix;
         SkString sk_TransformedCoords2D_0 = fragBuilder->ensureCoords2D(args.fTransformedCoords[0]);
-        fragBuilder->codeAppendf("half t = half(%s.x);\n%s = half4(t, 1.0, 0.0, 0.0);\n",
-                                 sk_TransformedCoords2D_0.c_str(), args.fOutputColor);
+        fragBuilder->codeAppendf(
+                "half t = half(%s.x) + 1.0000000000000001e-05;\n%s = half4(t, 1.0, 0.0, 0.0);\n",
+                sk_TransformedCoords2D_0.c_str(), args.fOutputColor);
     }
 
 private:

src/gpu/gradients/GrLinearGradientLayout.fp

@@ -12,7 +12,14 @@ in half4x4 gradientMatrix;
 }
 
 void main() {
-    half t = half(sk_TransformedCoords2D[0].x);
+    // We add a tiny delta to t. When gradient stops are set up so that a hard stop in a vertically
+    // or horizontally oriented gradient falls exactly at a column or row of pixel centers we can
+    // we can get slightly different interpolated t values along the column/row. By adding the delta
+    // we will consistently get the color to the "right" of the stop. Of course if the hard stop
+    // falls at X.5 - delta then we still could get inconsistent results, but that is much less
+    // likely. crbug.com/938592
+    // If/when we add filtering of the gradient this can be removed.
+    half t = half(sk_TransformedCoords2D[0].x) + 0.00001;
     sk_OutColor = half4(t, 1, 0, 0); // y = 1 for always valid
 }