Support non-black transparency in QPixmap and QPainter

Currently QPixmap and QPainter always paint fully transparent pixels as transparent black (0x00000000). Also, QPixmap::fromImage() converts existing non-black transparency into black, if Qt::NoFormatConversion is not specified (and even after specifying it, any transparent drawing in the pixmap still results in zero RGB channels).

While color channel for transparent pixels is usually not important, there are some valid cases (e.g. scaling/antialiasing when using the resulting image as a Direct3D texture) when saving transparent background with a specific color is critical. So, it would be great if Qt supported this at least with an optional flag.