Colored Emoji Font Support

This CL adds Emoji font support, which is needed for Chrome. Changes taken from 
https://chromium.googlesource.com/external/ots.git/+/6462c58fa123bb89baf1ec4b7538d8f2c682dc8b
This is done as per discussion with Behdad.
I think this is the only place where OTS needs to be changed. Clients will need to be adapted as descriped in #9 comments. The checks needs to be rewritten so that it checks that the input font has the two tables and that we are passing them through. Anyway this commit does not seem to be compilable (from the look of it, I didn't actually try).