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

Source/platform/graphics/ImageBuffer.cpp

Index: Source/platform/graphics/ImageBuffer.cpp
diff --git a/Source/platform/graphics/ImageBuffer.cpp b/Source/platform/graphics/ImageBuffer.cpp
index f37ac2bb71c84c90e8fe4aedfefdaf731fbd53b4..fbba10befe101257b532e3a883d45f7acc67d72f 100644
--- a/Source/platform/graphics/ImageBuffer.cpp
+++ b/Source/platform/graphics/ImageBuffer.cpp
@@ -396,7 +396,10 @@ static bool encodeImage(T& source, const String& mimeType, const double* quality
         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");
     }
@@ -411,6 +414,7 @@ String ImageBuffer::toDataURL(const String& mimeType, const double* quality) con
     Vector<char> encodedImage;
     if (!isSurfaceValid() || !encodeImage(m_surface->bitmap(), mimeType, quality, &encodedImage))
         return "data:,";
+
     Vector<char> base64Data;
     base64Encode(encodedImage, base64Data);