Font conguration for embedded fonts for the software renderer on MCU

[ogoffart]

When compiling with slint_build::EmbedResourcesKind::EmbedForSoftwareRenderer, we need a way to specify with font to embed, which glyph ranges, which sizes.

Currently there is some code that tries to auto-detect the sizes, and there is also the undocumented SLINT_FONT_SIZES env variable. But embedding all the fonts with all the glyphs with all size variant is taking way too much flash and is not practical.

[ogoffart]

Some options: (We can combine several approach)

1. Have a config file somewhere (see https://github.com/slint-ui/slint/issues/267 )
2. Some more configuration in slint_build
3. Add some syntax to the import "foo.ttf"; statement so we can specify sizes and ranges
4. Improve the flexibility of environment variables, and document them.

[MiSimon]

Would it be possible to extend Platform or add another Trait with a call like get_glyph(...) -> Option<...>? If a glyph is not found in the embedded font data, slint asks for the glyph.

[ogoffart]

There is another issue for loading font at runtime: https://github.com/slint-ui/slint/issues/4250 But a fallback in the platform seems like a good idea as well.