SkPDF: detect YUV-JPEG without relying on ImageGenerator

src/pdf/SkJpegInfo.h

+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+#ifndef SkJpegInfo_DEFINED
+#define SkJpegInfo_DEFINED
+
+class SkData;
+
+struct SkJFIFInfo {
+    int fWidth;
+    int fHeight;
+    enum Type {
+        kGrayscale,
+        kYCbCr,
+    } fType;
+};
+
+/** Returns true iff the data seems to be a valid JFIF JPEG image.  
+    If so and if info is not NULL, populate info.
+
+    JPEG/JFIF References:
+        http://www.w3.org/Graphics/JPEG/itu-t81.pdf
+        http://www.w3.org/Graphics/JPEG/jfif3.pdf
+*/
+bool SkIsJFIF(const SkData* skdata, SkJFIFInfo* info);
+
+#endif  // SkJpegInfo_DEFINED