Cancel pose stand when quitting.

This fixes the bug that if you Quit while in the Singularity -> Pose
Stand, then you are permanently hovering over the group until you go
into Advanced -> Debug Settings and reset AscentAvatarZModifier.

It also fixes that before, when you changed AscentAvatarZModifier
in Advanced -> Debug Settings then you saw no effect (until the
viewer would sent an AvatarAppearance message.

After this patch, changing any of the AscentAvatar*Modifier settings
has immediate visible effect, and a pose stand (Z-offset) is reset
when quitting.

indra/newview/llappviewer.cpp

@@ -1122,8 +1122,12 @@ bool LLAppViewer::mainLoop()
 	return true;
 }
 
+extern void cleanup_pose_stand(void);
+
 bool LLAppViewer::cleanup()
 {
+	cleanup_pose_stand();
+
 	//flag all elements as needing to be destroyed immediately
 	// to ensure shutdown order
 	LLMortician::setZealous(TRUE);