Blender Git Loki

Small constraint loop detection and calculation fix that now allows two objects to track each other (normal track or LockTrack).
This is done by disabling the object position refresh call when calculating these constraints. From the tests I did, this doesn't cause any problem at all.

The main point of this is to create pistons and the like.

For coders:

the detect_constraint_loop function now takes an additional parameter that determines the constraint type of the object it's looping from.