Duplicate event listeners added for animation events

Background: The parsing of the begin and end attributes of animations is 
tolerant to forward references and elements added later by allowing for a retry 
of the parsing upon certain events. This is complicated by the fact that begin 
targets and end targets can be different and resolved at different times.

The current implementation is simplistic in that it just keeps parsing 
everything until it resolves everything. Unfortunately, this can result in 
event listeners being added more than once for the target nodes as they are 
resolved. The resulting events listeners are being called more than once, 
resulting in excessive work being done.

The solution is to keep track of whether targets have been resolved already, 
individually, and only add the event listeners once.

Original issue reported on code.google.com by grick23@gmail.com on 20 Nov 2010 at 10:06

MRchildNEO / svgweb

Duplicate event listeners added for animation events #578