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.
be623e257f