[GTK][WPE] Stuttering when playing WebP animations

https://bugs.webkit.org/show_bug.cgi?id=297393

Reviewed by Carlos Garcia Campos.

When clearing the FrameBuffer cache, keep the most recent complete frame
around, because that will probably be necessary to decode the next frame.
This avoids having to decode several frames in a row in order to get the
next frame after a call to clearFrameBufferCache() is performed.

* Source/WebCore/platform/image-decoders/webp/WEBPImageDecoder.cpp:
(WebCore::WEBPImageDecoder::clearFrameBufferCache):

Canonical link: https://commits.webkit.org/298831@main

Author: magomez

Source/WebCore/platform/image-decoders/webp/WEBPImageDecoder.cpp

@@ -351,8 +351,13 @@ void WEBPImageDecoder::clearFrameBufferCache(size_t clearBeforeFrame)
     //
     // In WEBP every frame depends on the previous one or none. That means that frames after clearBeforeFrame
     // won't need any frame before them to render, so we can clear them all.
+    bool skipNextComplete = true;
     for (int i = clearBeforeFrame - 1; i >= 0; i--) {
         auto& buffer = m_frameBufferCache[i];
+        if (skipNextComplete && buffer.isComplete()) {
+            skipNextComplete = false;
+            continue;
+        }
         if (!buffer.isInvalid())
             buffer.clear();
     }