The Subsetter class now uses a module-level 'log', and no longer uses
its own custom Logger. This has been removed, and its features replaced
by equivalents.
log.lapse() are replaced with Timer context manager/decorator (these should make clearer where the elapsed time starts/stops);
log.glyphs() is kept for convenience: it is bound dynamically to the logger instance instead of subclassing logging.Logger, as it's only
useful in here.
log.font() is replaced by `font.saveXML(sys.stdout)`.
A distinct sub-logger is configured for timing messages, to allow --timing option to be enabled independently from --verbose
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.
