As needed in my Monotype OpenType format parser. I'm not using the object
directly from subset module, but pushing back my changes here, in the hope
that we move this object to ttLib proper soon.
Add support for subsetting COLR table.
The CPAL table does not need subsetting, but unused palettes should be
pruned, this is not implemented however as it depends on COLR table,
which in turn will not be updated after pruning CPAL.
