update skpdiff visualization (image magnification with alpha mask)

tools/skpdiff/SkDifferentPixelsMetric_opencl.cpp

Index: tools/skpdiff/SkDifferentPixelsMetric_opencl.cpp
diff --git a/tools/skpdiff/SkDifferentPixelsMetric_opencl.cpp b/tools/skpdiff/SkDifferentPixelsMetric_opencl.cpp
index f2a02c1c1823972793a9af7685910e6d46ec4d06..b3f5d2d7e0b00c70fb813e5b73ca4df3b5b9430d 100644
--- a/tools/skpdiff/SkDifferentPixelsMetric_opencl.cpp
+++ b/tools/skpdiff/SkDifferentPixelsMetric_opencl.cpp
@@ -47,6 +47,10 @@ const char* SkDifferentPixelsMetric::getName() {
     return "different_pixels";
 }
 
+bool SkDifferentPixelsMetric::enablePOIAlphaMask() {
+    return false;
+}
+
 int SkDifferentPixelsMetric::queueDiff(SkBitmap* baseline, SkBitmap* test) {
     int diffID = fQueuedDiffs.count();
     double startTime = get_seconds();