The project seems to be resuming activity (active), that's good, but there is a fundamental problem there is no official release, you mentioned a tar.gz package so that package maintainers can package it, this makes maintenance and updating easier.
During the "hibernation" of the project, forks emerged, some more active than others, in addition to distros and O.S. trying solutions or using old versions of AGG.
AGG Forks:
https://github.com/ghaerr/agg-2.6
https://github.com/aggeom/agg-2.6
Debian solution:
https://metadata.ftp-master.debian.org/changelogs//main/a/agg/agg_2.6.1-r134+dfsg1-2_changelog
Looking at fork commits, help the project find bugs and fixes!
To add to this, the fact that the SVN switched ot a 2.7 release (in r134) but then there never was a formal release of that (the repository has no tags, and there is no released source archive) is also a little confusing. Should we assume r134 is the 2.7.0 release?