Fix group delete bug, and improve friendliness to GC delete by removing references earlier.
Fix group delete bug, and improve friendliness to GC delete by removing
references earlier.

mpasternak/pyglet-fix-issue-552,gdkar/pyglet,mpasternak/pyglet-fix-issue-552,shaileshgoogler/pyglet,odyaka341/pyglet,kmonsoor/pyglet,gdkar/pyglet,arifgursel/pyglet,xshotD/pyglet,Austin503/pyglet,Alwnikrotikz/pyglet,shaileshgoogler/pyglet,arifgursel/pyglet,Alwnikrotikz/pyglet,mpasternak/michaldtz-fix-552,qbektrix/pyglet,shaileshgoogler/pyglet,Alwnikrotikz/pyglet,Austin503/pyglet,mpasternak/michaldtz-fix-552,mpasternak/michaldtz-fixes-518-522,gdkar/pyglet,odyaka341/pyglet,mpasternak/michaldtz-fixes-518-522,Alwnikrotikz/pyglet,mpasternak/michaldtz-fix-552,kmonsoor/pyglet,mpasternak/michaldtz-fix-552,Austin503/pyglet,mpasternak/pyglet-fix-issue-552,qbektrix/pyglet,arifgursel/pyglet,odyaka341/pyglet,arifgursel/pyglet,cledio66/pyglet,qbektrix/pyglet,mpasternak/michaldtz-fixes-518-522,mpasternak/michaldtz-fixes-518-522,xshotD/pyglet,shaileshgoogler/pyglet,cledio66/pyglet,gdkar/pyglet,shaileshgoogler/pyglet,google-code-export/pyglet,mpasternak/pyglet-fix-issue-552,xshotD/pyglet,google-code-export/pyglet,mpasternak/pyglet-fix-issue-518-522,kmonsoor/pyglet,mpasternak/pyglet-fix-issue-518-522,mpasternak/pyglet-fix-issue-518-522,Austin503/pyglet,Alwnikrotikz/pyglet,google-code-export/pyglet,odyaka341/pyglet,kmonsoor/pyglet,kmonsoor/pyglet,Austin503/pyglet,cledio66/pyglet,cledio66/pyglet,arifgursel/pyglet,gdkar/pyglet,mpasternak/pyglet-fix-issue-518-522,xshotD/pyglet,odyaka341/pyglet,cledio66/pyglet,google-code-export/pyglet,qbektrix/pyglet,google-code-export/pyglet,qbektrix/pyglet,xshotD/pyglet