Make swizzler optional for SkPngCodec, refactor xforms

src/codec/SkPngCodec.h

Index: src/codec/SkPngCodec.h
diff --git a/src/codec/SkPngCodec.h b/src/codec/SkPngCodec.h
index e4df8230766a53f6928a3f750fb6044e28674f14..24d54072f3f9282bbd0e8f02d3e02e76a4fc377c 100644
--- a/src/codec/SkPngCodec.h
+++ b/src/codec/SkPngCodec.h
@@ -46,13 +46,12 @@ protected:
     uint32_t onGetFillValue(SkColorType) const override;
 
     // Helper to set up swizzler, color xforms, and color table. Also calls png_read_update_info.
-    bool initializeXforms(const SkImageInfo& requestedInfo, const Options&, SkPMColor* colorPtr,
+    bool initializeXforms(const SkImageInfo& dstInfo, const Options&, SkPMColor* colorPtr,
                           int* colorCount);
-    SkSampler* getSampler(bool createIfNecessary) override {
-        SkASSERT(fSwizzler);
-        return fSwizzler;
-    }
-    void allocateStorage();
+    void initializeSwizzler(const SkImageInfo& dstInfo, const Options&);
+    SkSampler* getSampler(bool createIfNecessary) override;
+    void allocateStorage(const SkImageInfo& dstInfo);
+    void applyXformRow(void* dst, const void* src, SkColorType, SkAlphaType, int width);
 
     virtual int readRows(const SkImageInfo& dstInfo, void* dst, size_t rowBytes, int count,
                          int startRow) = 0;
@@ -77,8 +76,22 @@ protected:
     int                                fBitDepth;
 
 private:
+
+    enum XformMode {

[msarett, 2016/08/26 16:37:45]

There are two more cases where we are not quite optimal.
(1) When we could decode directly to dst and skip swizzling and color xforms.
(2) When we could perform the color xform in place (don't need an extra row
buffer).

I considered adding both of these modes, but ultimately decided the gain wasn't
worth the complexity.

+        // Requires only a swizzle pass.
+        kSwizzleOnly_XformMode,
+
+        // Requires only a color xform pass.
+        kColorOnly_XformMode,
+
+        // Requires a swizzle and a color xform.
+        kSwizzleColor_XformMode,
+    };
+
     bool createColorTable(const SkImageInfo& dstInfo, int* ctableCount);
     void destroyReadStruct();
 
+    XformMode                          fXformMode;
+
     typedef SkCodec INHERITED;
 };