No default fonts for egui exposed by egui_winit

When using `egui_winit` and using the exposed `egui_winit::egui` crate to create the egui context, no default font is bundled. This leads to the `tesselate` step outputting no text.

This is somewhat expected since [the egui_winit toml](https://github.com/emilk/egui/blob/master/crates/egui-winit/Cargo.toml#L46) declares `default-features=false` but does not include the `default_fonts` feature. However, this is unexpected when writing an egui integration, since most backends that are already implemented use the default fonts.

I would send a pull request, but I am not sure what the correct way would be. It makes sense to use `egui_winit::egui` for a egui integration that uses winit as well (to keep the versions compatible). Maybe a warning at the top-level documentation is enough. Something like *When using `egui_winit::egui` default features are disabled* might be enough.

Start with crates/egui-winit/Cargo.toml, especially its egui dependency features, and the top-level egui_winit documentation. Reproduce the integration using egui_winit::egui and check whether the default_fonts feature is absent and text disappears during tessellation. Done means documenting the feature choice clearly or establishing the appropriate dependency-feature change.

Written by the indexing model from the issue text.