Add Alpha8 support to SkPNGImageEncoder

src/codec/SkPngCodec.cpp

Index: src/codec/SkPngCodec.cpp
diff --git a/src/codec/SkPngCodec.cpp b/src/codec/SkPngCodec.cpp
index 2c13b12c7e75ffa00a3e04da067def657fa35141..853334d71b635d728a775e87e604ffa9eba2bcbf 100644
--- a/src/codec/SkPngCodec.cpp
+++ b/src/codec/SkPngCodec.cpp
@@ -764,6 +764,18 @@ static bool read_header(SkStream* stream, SkPngChunkReader* chunkReader, SkCodec
             }
         }
 
+        if (PNG_COLOR_TYPE_GRAY == encodedColorType) {
+            png_textp text;
+            if (png_get_text(png_ptr, info_ptr, &text, nullptr)) {
+                if (0 == strcmp("SkColorType", text->key) && 0 == strcmp("Alpha8", text->text)) {

[msarett, 2016/08/30 14:30:38]

Ideally, we store these strings somewhere that is shared by the decoder and
encoder... Any ideas?

+                    // Recommend a decode to Alpha8 if our custom encoder has marked
+                    // the image as Alpha8.
+                    imageInfo = imageInfo.makeColorType(kAlpha_8_SkColorType)
+                                         .makeAlphaType(kPremul_SkAlphaType);
+                }
+            }
+        }
+
         if (1 == numberPasses) {
             *outCodec = new SkPngNormalCodec(encodedInfo, imageInfo, stream,
                     chunkReader, png_ptr, info_ptr, bitDepth);