Merge branch 'hotfix-3.0.1'
Merge branch 'hotfix-3.0.1'
Revert "VirtualGL as default, primus as alternative"

This reverts commit a22b602ebb345a9bba4684970a3487ffd2a45b45.

Since this branch will only be merged when primus will be ready for replacing VirtualGL as default, there is no need for this.
Added 3.1 release notes.

See #318 for details.
Remove old stuff from release notes, add new info
Update configure.ac version number
Merge branch 'release-3.1'
Merge branch 'common-primus' into develop