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.

theblacklion/pyglet,oktayacikalin/pyglet,theblacklion/pyglet,mammadori/pyglet,mammadori/pyglet,oktayacikalin/pyglet,theblacklion/pyglet,oktayacikalin/pyglet,mammadori/pyglet,theblacklion/pyglet,theblacklion/pyglet,oktayacikalin/pyglet,mammadori/pyglet,oktayacikalin/pyglet