Instead of directly accessing constraint-specific callbacks
in code all over blender, introduce two wrappers to retrieve
and free the target list.
This incidentally revealed a place within the Collada exporter
in BCAnimationSampler.cpp that didn't clean up after retrieving
the targets, resulting in a small memory leak. Fixing this should
be the only functional change in this commit.
This was split off from D9732.
Differential Revision: https://developer.blender.org/D13844
d040e1da4f