Emoji rendering on macOS

This requires adding support for color (ARGB) glyphs to glyph rendering pipeline at all its stages (generation, storage and blitting). Emoji glyphs on macOS are provided by 'Apple Color Emoji' system font. It cannot be rendered with Core Graphics API, Core Text should be used instead. As the font is a bitmap one, and it's not scaled proportionally with font size, special measures should be taken to perform its correct sizing during rendering and metrics calculation.