Introduce three compilation modes:
1. Pure python: only uses the existing fonttools packing and overflow resolution.
2. Harfbuzz+python: uses harfbuzz packing and python overflow resolution. Extensions are allowed to be shared.
3. Python fallback: if harfbuzz+python runs out of resolution options, this disables extension sharing and only uses python packing. Once it succeeds control is passed back to the harfbuzz packer to produce the final packing with extension sharing enabled.
this will be useful when computing deltas for COLR table fields that use Fixed or F2Dot14 (ItemVariationStore internally treats fixed-value floats as 16 or 32-bit integers)
though we still emit the empty <Map index=.../> elements; they help identify chunks of delta-set indices, and define the length of the array upon loading from XML
we tell GzipFile to write the MTIME field to zero so that the compressed output is reproducible and doesn't change depending on when the data is compressed.
This is based on bungeman's https://github.com/fonttools/fonttools/pull/2627
Previously, an entire `SVG ` table would be marked as compressed if any
of the decoded SVG documents in it were compressed. Then on encoding all
SVG documents would be considered for compression. The XML format had no
means to indicate if compression was desired.
Instead, mark each svgDoc with its compression status. When decoding
mark the svgDoc as compressed if the data was compressed. When encoding
try to compress the svgDoc if it is marked as compressed. In the XML
format the data itself is always uncompressed, but allow an optional
`compressed` boolean attribute (defaults to false) to indicate the
svgDoc should be compressed when encoded.
We also try to make sure that older code that relies on docList containing
sequences of three items (doc, startGID, endGID) will continue to work
without modification.
Harfbuzz repacker can handle subtables shared across extension lookups so there's no need to disallow such sharing. Add a parameter to _doneWriting to allow sharing to be enabled when preparing the object graph for passing to harfbuzz.
if explicitly enabled, it will raise ImportError if uharfbuzz is not found, and will propagate the uharfbuzz error instead of silently falling back to the pure-python serializer
