Implement approx-match support in image filter saveLayer() offscreen.

src/gpu/effects/GrTextureDomain.h

Index: src/gpu/effects/GrTextureDomain.h
diff --git a/src/gpu/effects/GrTextureDomain.h b/src/gpu/effects/GrTextureDomain.h
index 7efd9f2dee1850d1d054582be3e577b9df5ff829..6a462f96a8c77bdbce1396783679fb92122d3a17 100644
--- a/src/gpu/effects/GrTextureDomain.h
+++ b/src/gpu/effects/GrTextureDomain.h
@@ -69,6 +69,19 @@ public:
         return result;
     }
 
+    static const SkRect MakeTexelDomainForMode(const GrTexture* texture, const SkIRect& texelRect, Mode mode) {
+        // For Clamp mode, inset by half a texel.
+        SkScalar wInv = SK_Scalar1 / texture->width();
+        SkScalar hInv = SK_Scalar1 / texture->height();
+        SkScalar inset = (mode == kClamp_Mode && !texelRect.isEmpty()) ? SK_ScalarHalf : 0;
+        return SkRect::MakeLTRB(
+            (texelRect.fLeft + inset) * wInv,
+            (texelRect.fTop + inset) * hInv,
+            (texelRect.fRight - inset) * wInv,
+            (texelRect.fBottom - inset) * hInv
+        );
+    }
+
     bool operator== (const GrTextureDomain& that) const {
         return fMode == that.fMode && (kIgnore_Mode == fMode || fDomain == that.fDomain);
     }