[macOS] Poor rendering by QCoreTextFontEngine on popup windows

This is particularly visible on non-retina displays. See attached screenshots.

The picture below is how QCoreTextFontEngine renders text when invoked via QPainter in QMacStyle. Notice, in particular, the poor rendition of the uppercase "U" in "Undo". In general, the glyphs stems look bolder than they should and maybe off the pixel grid.

When we delegate the rendering to AppKit instead, which is also using CoreText (see below), the glyphs shapes, their stems in particular, are much smoother and feel much better aligned to the pixel grid. We can also notice a difference in width for the rendered text.

See https://codereview.qt-project.org/216093 for a workaround in QMacStyle.