Function render 

pub fn render(
    rules: &GlyphTiles,
    atlas: &FontAtlasDef,
    atlas_image: &Image,
    palette: &PaletteDef,
) -> Result<Image, String>

Draws the tileset rules describes with the font atlas atlas (its picture atlas_image) and the colors of palette. Fails on a color the palette lacks, a glyph the atlas lacks, or a tile of no glyphs, too many, or too many to fit.