Remove downwards bias when shading gradients

src/effects/gradients/SkLinearGradient.cpp

Index: src/effects/gradients/SkLinearGradient.cpp
diff --git a/src/effects/gradients/SkLinearGradient.cpp b/src/effects/gradients/SkLinearGradient.cpp
index 4f85da39d378b260bfd883fa0749e9a6f53599c5..6f635460005a10097b5fec60b76257ba25d088bc 100644
--- a/src/effects/gradients/SkLinearGradient.cpp
+++ b/src/effects/gradients/SkLinearGradient.cpp
@@ -217,7 +217,7 @@ void SkLinearGradient::LinearGradientContext::shadeSpan(int x, int y, SkPMColor*
     if (fDstToIndexClass != kPerspective_MatrixClass) {
         dstProc(fDstToIndex, SkIntToScalar(x) + SK_ScalarHalf,
                              SkIntToScalar(y) + SK_ScalarHalf, &srcPt);
-        SkFixed dx, fx = SkScalarToFixed(srcPt.fX);
+        SkFixed dx, fx = SkScalarToFixed(srcPt.fX) - (SK_FixedHalf >> kCache32Bits);
 
         if (fDstToIndexClass == kFixedStepInX_MatrixClass) {
             SkFixed dxStorage[1];
@@ -404,7 +404,7 @@ void SkLinearGradient::LinearGradientContext::shadeSpan16(int x, int y,
     if (fDstToIndexClass != kPerspective_MatrixClass) {
         dstProc(fDstToIndex, SkIntToScalar(x) + SK_ScalarHalf,
                              SkIntToScalar(y) + SK_ScalarHalf, &srcPt);
-        SkFixed dx, fx = SkScalarToFixed(srcPt.fX);
+        SkFixed dx, fx = SkScalarToFixed(srcPt.fX) - (SK_FixedHalf >> kCache32Bits);
 
         if (fDstToIndexClass == kFixedStepInX_MatrixClass) {
             SkFixed dxStorage[1];