Direct composite canvas background if possible.

Source/core/platform/graphics/gpu/DrawingBuffer.cpp

 /*
  * Copyright (c) 2010, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 288 matching lines @@
 
 WebKit::WebLayer* DrawingBuffer::platformLayer()
 {
     if (!m_context)
         return 0;
 
     if (!m_layer) {
         m_layer = adoptPtr(WebKit::Platform::current()->compositorSupport()->createExternalTextureLayer(this));
 
         m_layer->setOpaque(!m_attributes.alpha);
+        m_layer->setBlendBackgroundColor(m_attributes.alpha);
         m_layer->setPremultipliedAlpha(m_attributes.premultipliedAlpha);
         GraphicsLayer::registerContentsLayer(m_layer->layer());
     }
 
     return m_layer->layer();
 }
 
 void DrawingBuffer::paintCompositedResultsToCanvas(ImageBuffer* imageBuffer)
 {
     if (!m_context || !m_context->makeContextCurrent() || m_context->getExtensions()->getGraphicsResetStatusARB() != GraphicsContext3D::NO_ERROR)
@@ skipping 409 matching lines @@
 
 void DrawingBuffer::bind()
 {
     if (!m_context)
         return;
 
     m_context->bindFramebuffer(GraphicsContext3D::FRAMEBUFFER, m_multisampleFBO ? m_multisampleFBO : m_fbo);
 }
 
 } // namespace WebCore