Build color fonts

It would be nice if fontmake could build color fonts.

Glyphs already supports color fonts, both as [vectors](https://glyphsapp.com/tutorials/creating-a-microsoft-color-font) and [images](https://glyphsapp.com/tutorials/creating-an-apple-color-font). Therefore, a glyphs file should contain all information needed to build color fonts in various formats.

Sadly, color font formats are very fragmented. As a general tool chain, I think fontmake should be agnostic about formats and try to support them all. (Of course, it would be nice if the industry could converge eventually). As an intermediate format, SVG might work because it can be easily converted into PDF, PNG, and JPEG images. Later in the pipeline, those images can then be packaged into [sbix](https://developer.apple.com/fonts/TrueType-Reference-Manual/RM06/Chap6sbix.html), [CBDT](https://www.microsoft.com/typography/otspec/cbdt.htm), [COLR](https://www.microsoft.com/typography/otspec/colr.htm), [CBDT/CBLC](https://github.com/googlei18n/noto-emoji/blob/master/third_party/color_emoji/emoji_builder.py), and similar tables depending on what format was requested.