Enable with `SLINT_EMBED_GLYPHS=1` and select sizes like
`SLINT_FONT_SIZES=12,16`
This change just puts the data structures in place, rasterizes a fixed
subset, embeds that into the rust generated code and calls a backend
function for registering the font that is unimplemented.
Move "internal" crates into the `internal` directory. This first batch
includes most of sixtyfps_runtime but leaves the rendering backends
alone for now.
pre-commit applied some cleanups to the moved files:
- Consistent newline at end of file policy
- trimming trailing whitespace
- Formatting Cargo.toml files.
