Add support for multiple frames in SkCodec

src/codec/SkStreamBuffer.h

+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkStreamBuffer_DEFINED
+#define SkStreamBuffer_DEFINED
+
+#include "SkStream.h"
+#include "SkTypes.h"
+
+/**
+ *  Helper class for reading from a stream that may not have all its data
+ *  available yet.
+ *
+ *  Used by GIFImageReader, and currently set up for that use case.
+ *
+ *  Buffers up to 256 * 3 bytes (256 colors, with 3 bytes each) to support GIF.
+ *  FIXME (scroggo): Make this more general purpose?
+ */
+class SkStreamBuffer : SkNoncopyable {
+public:
+    // Takes ownership of the SkStream.
+    SkStreamBuffer(SkStream*);
+
+    /**
+     *  Return a pointer the buffered data.
+     *
+     *  The number of bytes buffered is the sum of values returned by calls to
+     *  buffer() since the last call to flush().
+     */
+    const char* get() const { SkASSERT(fBytesBuffered >= 1); return fBuffer; }
+
+    /**
+     *  Bytes in the buffer.
+     *
+     *  Sum of the values returned by calls to buffer() since the last call to
+     *  flush().
+     */
+    size_t bytesBuffered() const { return fBytesBuffered; }
+
+    /**
+     *  Buffer from the stream into our buffer.
+     *
+     *  Returns the number of bytes successfully buffered.
+     */
+    size_t buffer(size_t bytes);
+
+    /**
+     *  Flush the buffer.
+     *
+     *  After this call, no bytes are buffered.
+     */
+    void flush() {
+        fBytesBuffered = 0;
+    }
+
+private:
+    static constexpr size_t kMaxSize = 256 * 3;
+
+    std::unique_ptr<SkStream>   fStream;
+    char                        fBuffer[kMaxSize];
+    size_t                      fBytesBuffered;
+};
+#endif // SkStreamBuffer_DEFINED
+