[XeTeX] Support of variable OpenType fonts

[Werner LEMBERG]

> May I /very/ respectfully suggest that while support for colour emoji
> fonts may well be of interest to some, it would be more in the spirit
> of TeX to invest that effort into attempting to support the technology
> that underlies Zapfino Ink
> <https://tug.org/TUGboat/tb28-2/tb89hagen.pdf>.

Essentially, it's the same technology for both: Either OpenType 'COLR'
v1 tables

  https://docs.microsoft.com/en-us/typography/opentype/spec/colr

or 'SVG' tables.

  https://docs.microsoft.com/en-us/typography/opentype/spec/svg


    Werner