Use Qt::transparent to resolve graphical artifacts with images with transparent background.

We need to use QImage::Format_ARGB32_Premultiplied on Windows to preserve a text hinting and antialiasing. Using the function `qRgba()` leads to wrong pixel values on a target image. Since, `QImage::fill(uint pixel)` doesn't handle the QImage::Format_ARGB32_Premultiplied format, so we need to use the another overload `QImage::fill(const QColor &color)`

Issues:
https://github.com/ariya/phantomjs/issues/11276
https://github.com/ariya/phantomjs/issues/11007
https://github.com/ariya/phantomjs/issues/11366