Animated WebP: Optimize decoding in case of seeking

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

Index: Source/core/platform/image-decoders/ImageDecoder.cpp
diff --git a/Source/core/platform/image-decoders/ImageDecoder.cpp b/Source/core/platform/image-decoders/ImageDecoder.cpp
index 917161f10527eb91c1636fd73868dfe04515499f..db18ed09280479e0719b72675ecee755b285321d 100644
--- a/Source/core/platform/image-decoders/ImageDecoder.cpp
+++ b/Source/core/platform/image-decoders/ImageDecoder.cpp
@@ -152,7 +152,7 @@ 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) {
@@ -160,6 +160,11 @@ size_t ImageDecoder::findRequiredPreviousFrame(size_t frameIndex)
         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;