duplicate of BKE_curframe() which just takes two extra args.
For the few calls in the physics engine where CFRA+1 instead of CFRA
was being used, I've added a new BKE_nextframe() call, which will
calculate for CFRA+1 instead of CFRA in much the same way that
bsystem_time() would end up doing things (which means including
subframe steps).