DM: add a pixel check to serialize

dm/DMSrcSink.cpp

Index: dm/DMSrcSink.cpp
diff --git a/dm/DMSrcSink.cpp b/dm/DMSrcSink.cpp
index fa600ff4eb33bc07ba22c6d0eebe46e435085b52..82962ff6920e873eb54ea6c7751e88368557ef19 100644
--- a/dm/DMSrcSink.cpp
+++ b/dm/DMSrcSink.cpp
@@ -1084,6 +1084,15 @@ Error ViaRemote::draw(const Src& src, SkBitmap* bitmap, SkWStream* stream, SkStr
 
 Error ViaSerialization::draw(
         const Src& src, SkBitmap* bitmap, SkWStream* stream, SkString* log) const {
+    // Draw the Src directly as a reference.
+    SkBitmap reference;
+    if (bitmap) {
+        Error err = fSink->draw(src, &reference, nullptr, log);
+        if (!err.isEmpty()) {
+            return err;
+        }
+    }
+
     // Record our Src into a picture.
     auto size = src.size();
     SkPictureRecorder recorder;
@@ -1102,6 +1111,15 @@ Error ViaSerialization::draw(
 
     return draw_to_canvas(fSink, bitmap, stream, log, size, [&](SkCanvas* canvas) {
         canvas->drawPicture(deserialized);
+        // Check against the reference if we have one.
+        if (bitmap) {
+            if (reference.getSize() != bitmap->getSize()) {
+                return "Serialized and direct have different dimensions.";
+            }
+            if (0 != memcmp(reference.getPixels(), bitmap->getPixels(), reference.getSize())) {
+                return "Serialized and direct have different pixels.";
+            }
+        }
         return "";
     });
 }