[ENH] Support for SBIX-encoded colour glyphs #31208

Colour glyphs using the SBIX tables are PNG/JPEG/TIFF images embedded instead of vector glyphs, using tables designed by Apple. FreeType only supports PNG here, with the png option linking to libpng that we don't currently enable. FreeType has some example code for working with bitmap fonts.

Hopefully, as with CBDT/CBLC, we can extract the glyph data and pass it to Pillow (and thus support the other formats as well!)

The major font using this is likely the Apple Color Emoji font, at least based on the copy I found...