Animated WebP: Optimize decoding in case of seeking

Source/core/platform/image-decoders/ImageDecoder.cpp

 /*
  * Copyright (C) Research In Motion Limited 2009-2010. All rights reserved.
  *
  *  This library is free software; you can redistribute it and/or
  *  modify it under the terms of the GNU Library General Public
  *  License as published by the Free Software Foundation; either
  *  version 2 of the License, or (at your option) any later version.
  *
  *  This library is distributed in the hope that it will be useful,
  *  but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 134 matching lines @@
         }
     }
     return frameBytesCleared;
 }
 
 void ImageDecoder::clearFrameBuffer(size_t frameIndex)
 {
     m_frameBufferCache[frameIndex].clearPixelData();
 }
 
-size_t ImageDecoder::findRequiredPreviousFrame(size_t frameIndex)
+size_t ImageDecoder::findRequiredPreviousFrame(size_t frameIndex, bool frameRectIsOpaque)
 {
     ASSERT(frameIndex <= m_frameBufferCache.size());
     if (!frameIndex) {
         // The first frame doesn't rely on any previous data.
         return notFound;
     }
 
+    const ImageFrame* currBuffer = &m_frameBufferCache[frameIndex];
+    if ((frameRectIsOpaque || currBuffer->alphaBlendSource() == ImageFrame::BlendAtopBgcolor)
+        && currBuffer->originalFrameRect().contains(IntRect(IntPoint(), size())))
+        return notFound;
+
     // The starting state for this frame depends on the previous frame's
     // disposal method.
     size_t prevFrame = frameIndex - 1;
     const ImageFrame* prevBuffer = &m_frameBufferCache[prevFrame];
     ASSERT(prevBuffer->requiredPreviousFrameIndexValid());
 
     switch (prevBuffer->disposalMethod()) {
     case ImageFrame::DisposeNotSpecified:
     case ImageFrame::DisposeKeep:
         // prevFrame will be used as the starting state for this frame.
@@ skipping 13 matching lines @@
         // Otherwise, the previous frame contributes to this frame.
         return (prevBuffer->originalFrameRect().contains(IntRect(IntPoint(), size()))
             || (prevBuffer->requiredPreviousFrameIndex() == notFound)) ? notFound : prevFrame;
     default:
         ASSERT_NOT_REACHED();
         return notFound;
     }
 }
 
 } // namespace WebCore