[wip] Add canvas.toDataURL("image/png") compression control

Source/platform/graphics/ImageBuffer.cpp

 /*
  * Copyright (c) 2008, Google Inc. All rights reserved.
  * Copyright (C) 2009 Dirk Schulze <krit@webkit.org>
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. 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
@@ skipping 378 matching lines @@
             compressionQuality = static_cast<int>(*quality * 100 + 0.5);
         if (!JPEGImageEncoder::encode(source, compressionQuality, encodedImage))
             return false;
     } else if (mimeType == "image/webp") {
         int compressionQuality = WEBPImageEncoder::DefaultCompressionQuality;
         if (quality && *quality >= 0.0 && *quality <= 1.0)
             compressionQuality = static_cast<int>(*quality * 100 + 0.5);
         if (!WEBPImageEncoder::encode(source, compressionQuality, encodedImage))
             return false;
     } else {
-        if (!PNGImageEncoder::encode(source, encodedImage))
+        int compressionQuality = PNGImageEncoder::DefaultCompressionQuality;
+        if (quality && *quality >= 0.0 && *quality <= 1.0)
+            compressionQuality = static_cast<int>(*quality * 100 + 0.5);
+        if (!PNGImageEncoder::encode(source, compressionQuality, encodedImage))
             return false;
         ASSERT(mimeType == "image/png");
     }
 
     return true;
 }
 
 String ImageBuffer::toDataURL(const String& mimeType, const double* quality) const
 {
     ASSERT(MIMETypeRegistry::isSupportedImageMIMETypeForEncoding(mimeType));
 
     Vector<char> encodedImage;
     if (!isSurfaceValid() || !encodeImage(m_surface->bitmap(), mimeType, quality, &encodedImage))
         return "data:,";
+
     Vector<char> base64Data;
     base64Encode(encodedImage, base64Data);
 
     return "data:" + mimeType + ";base64," + base64Data;
 }
 
 String ImageDataToDataURL(const ImageDataBuffer& imageData, const String& mimeType, const double* quality)
 {
     ASSERT(MIMETypeRegistry::isSupportedImageMIMETypeForEncoding(mimeType));
 
     Vector<char> encodedImage;
     if (!encodeImage(imageData, mimeType, quality, &encodedImage))
         return "data:,";
 
     Vector<char> base64Data;
     base64Encode(encodedImage, base64Data);
 
     return "data:" + mimeType + ";base64," + base64Data;
 }
 
 } // namespace WebCore