[read-fonts] parse embedded bitmap tables

[dfrg]

This can now read Noto Color Emoji bitmaps with code like the following:

    let data = std::fs::read("path/to/NotoColorEmoji.ttf").unwrap();
    let font = FontRef::from_index(&data, 0).unwrap();
    let ch = '🫶';
    let ch = '✨';
    let ch = '\u{2764}';
    let cmap = font.cmap().unwrap();
    let cblc = font.cblc().unwrap();
    let cbdt = font.cbdt().unwrap();
    let gid = cmap.map_codepoint(ch).unwrap();
    let size = &cblc.bitmap_sizes()[0];
    let location = size.location(cblc.offset_data(), gid).unwrap();
    let bitmap = cbdt.data(&location).unwrap();
    std::fs::write("emoji_image.png", bitmap.data).unwrap();

[dfrg]

#660 for missing format tests