Summarized Changelog:
- Fix issue with dropping xlink:href attribute when collapsing referenced gradients.
- Fix issue with dropping ID while de-duplicating gradients.
- Improve --shorten-ids so it re-maps IDs that are already used in the document if they're shorter.
- Fix whitespace handling for SVG 1.2 flowed text.
- Improvement: Merge sibling <g> nodes with identical attributes.
- Improve performance of XML serialization.
- Improve performance of gradient de-duplication.
- Some general performance improvements.