Use the static (but long) fallback list that we get from fontconfig in the beginning.
This works, but can be optimized to operate on a trimmed and shorter list,
which will speed up the fallback. But for now this makes it work.
The basic idea is to use fontdb's load_system_fonts() mechanism, in
conjunction with its built-in mmap support, to get an overview over all
installed fonts on macOS, Windows and Linux.
This isn't quite perfect in terms of discovering systems defaults, but
it's much faster than font-kit's approach of querying the system (good)
but reading the matched files into memory (not using mmap). And we have
the option of perfecting it by using fontconfig directly on Linux (where
the backend is most important).
This also paves the way for better fallback handling, as now we have a
list of all available families and we can use system APIs to query for
fallbacks.
Our demos use some symbols such as the back arrow and the ugly style
uses a down arrow for the combo box. These symbols are not in Roboto and
also not in the default font font_kit gives us on macOS.
Therefore for WASM we switch over to DejaVu, which is bigger than
Roboto.
For macOS this patch implements rudimentary fallback list generation
using CTFontCopyDefaultCascadeListForLanguages. This needs further work
to respect the locale and correctly translate styles - eventually this
should go into font-kit.
A general aspect with the font fallback handling is that femtovg doesn't
support on-demand loading yet. Instead it uses owned_ttf_parser, so
every font will be read entirely off disk. That's why for macOS we trim
the list. We could perhaps do the analysis ourselves.
Linux and Windows are to follow.
