We populate the various QPlatformTheme::Fonts by asking the system for
the preferred font for each use-case, but that just gives us a single
font descriptor with a single style and weight. If the user then tweaks
the font by e.g. making it bold or italic, our font database will not
have any knowledge of these variants, and will fall back to another
font.
To fix this we ask CoreText for all variants of each of the theme fonts,
so that each of the theme font families are fully populated.
The preferred way to do this on macOS 10.15/iOS 13 and above is by using
the font's UI design trait. This avoids asking CoreText for private
fonts by family name directly, which is not supported and may result
in giving us Times or another fallback font instead.
Pick-to: 6.2 6.3
Change-Id: I4aa21624df62ac09a92d7ae0bc2cdde4f6ad6e5f
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
c7280b3362