Add support for BGRA glyphs and scaling

Scaling is done by averaging all relevant pixel, which gives much better result than nearest pixel sampling while staying simple enough and not too computationally expensive.

This enables color emoji rendering support.

I tested this on st and dmenu, both work fine once fontconfig is setup to prepend color emoji fonts. rxvt-unicode requires a small patch.

dmenu:

urxvt:

st: