Add support for row-by-row png encodes

Also adds a SkEncoder base class.

Bug: 713862
Change-Id: Ia3f009cd9f376514f6c19396245fab3a43ae6536
Reviewed-on: https://skia-review.googlesource.com/15152
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>
diff --git a/gm/encode-platform.cpp b/gm/encode-platform.cpp
index b6e0c1c..fa940b6 100644
--- a/gm/encode-platform.cpp
+++ b/gm/encode-platform.cpp
@@ -13,6 +13,7 @@
 #include "SkImage.h"
 #include "SkImageEncoderPriv.h"
 #include "SkJpegEncoder.h"
+#include "SkPngEncoder.h"
 #include "SkUnPreMultiply.h"
 
 namespace skiagm {
@@ -69,9 +70,12 @@
         return SkEncodeImageWithWIC(&buf, src, type, 100) ? buf.detachAsData() : nullptr;
     #else
         switch (type) {
-            case SkEncodedImageFormat::kPNG:
-                return SkEncodeImageAsPNG(&buf, src, SkEncodeOptions()) ? buf.detachAsData()
-                                                                        : nullptr;
+            case SkEncodedImageFormat::kPNG: {
+                SkPngEncoder::Options options;
+                options.fUnpremulBehavior = SkTransferFunctionBehavior::kIgnore;
+                bool success = SkPngEncoder::Encode(&buf, src, options);
+                return success ? buf.detachAsData() : nullptr;
+            }
             case SkEncodedImageFormat::kJPEG: {
                 bool success = SkJpegEncoder::Encode(&buf, src, SkJpegEncoder::Options());
                 return success ? buf.detachAsData() : nullptr;