Low resolution title bar icon on Wayland on Hi DPI displays

On my Linux/Wayland (Gnome) system, when the DPI scale is set to 200%, the window title bar icon of my Qt application is displayed in low resolution. I have provided an QIcon with an appropriate hi resolution pixmap to `QApplication::setWindowIcon`, therefore I would expect a full resolution result.

The responsible code appears to be here: https://github.com/qt/qtwayland/blame/50dbd5f508e03f1cc57300830049581bef9c73b2/src/plugins/decorations/bradient/main.cpp#L165-L175

QPixmap pixmap = icon.pixmap(QSize(128, 128));
QPixmap scaled = pixmap.scaled(22, 22, Qt::IgnoreAspectRatio, Qt::SmoothTransformation);

QRectF iconRect(0, 0, 22, 22);
p.drawPixmap(iconRect.adjusted(margins().left() + BUTTON_SPACING, 4,
                               margins().left() + BUTTON_SPACING, 4),
             scaled, iconRect);

In this snippet, it is apparent that the QIcon 128x128 (or 256x256) pixmap is explicitly scaled to 22x22 native pixels, and then painted using the display scale aware `QPainter` which will scale it back up to 44x44 for display. As with QTBUG-90363, the double scaling results in poor visual quality.

I suggest replacing the above code with the following:

icon.paint(&p, iconRect.adjusted(margins().left() + BUTTON_SPACING, 4,
                               margins().left() + BUTTON_SPACING, 4));

That way, QIcon uses directly a pixmap with the appropriate size according to the display scale.