wip: support render png colord emoji

[NoXF]

• GlyphBitmap changed to Enum and supported the type
• add bitmap_png function, to create GlyphBitmap::PNG
• add emoji_font_key used to determine whether char is an emoji
• Only support for NotoColorEmoji was tested

let mut fontkit = FontKit::new();
let emoji = FontKey::new_with_family("Noto Color Emoji".to_string());
fontkit.set_emoji(emoji);

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