On macOS and Windows, use winit's `Window::theme()` instead of
dark-light, to detect the theme.
Always react to WindowEvent::ThemeChange. It's delivered on macOS,
Windows, and the web.
The crate is an internal crate until the API has been polished and
documented, after which we can call slint-renderer-skia for example.
This also duplicates a little bit of the glutin setup code, because
that would otherwise have to go into another shared crate.
This allows setting the RUST_FONTCONFIG_DLOPEN environment variable
to dlopen fontconfig at runtime rather than linking it at build
time. This is helpful for cross compiling to Linux, particularly
because fontconfig has lots of C dependencies. Building a vendored
copy of fontconfig does not work as expected:
https://github.com/slint-ui/slint/issues/88
When opting into the Skia renderer, we default to metal on macOS and D3D on Windows.
However if you want to develop a cross-platform application with Skia and
for example rely on OpenGL to be able to implement an OpenGL underlay or overlay,
then we need the ability to explicitly opt into skia's GL renderer.
cc #1445
