Speed up of LLViewerObjectList::cleanDeadObjects with a factor of 10000.

Also a little review of Dead objects in general.
The old code was calling 'erase' on a large vector, once for
every dead object (which are a lot, especially if you exit or
teleport), causing the whole (large) vector to be copied every
time. The new code only calls erase once per call (about once
a second at most now), erasing 20 to 100 objects at the END
of the vector. This is INCREDIBLY faster.

indra/newview/floaterlocalassetbrowse.cpp

@@ -376,6 +376,12 @@ std::vector<affected_object> LocalBitmap::getUsingObjects(bool seek_by_type, boo
 		 iter != gObjectList.mObjects.end(); iter++ )
 	{
 		LLViewerObject* obj = *iter;
+
+		if (obj->isDead())
+		{
+			continue;
+		}
+
 		affected_object shell;
 		shell.object = obj;
 		shell.local_sculptmap = false;