Forward the text and color properties to the rendering backend, where
right now we just rendering all the glyphs into a dedicated texture.
Next steps are a glyph atlas texture, blending the specified color with
the alpha of the glyphs, configurable size and family, shaping with
Harfbuzz and may more things.
The source property is a path relative to the path of the executable.
That is still rather inconvenient. It would be more convenient to make
it relative to the source file, but the corresponding proc-macro span
source_file() accessor is not public/stable.
Perhaps there's a way of doing this also through the generated code and
std::file!().
