Ray branching and DOF-enabled scenes

#1559 adds the ability to reuse initial rays multiple times, which is beneficial to performance in many situations; however, the convergence of DOF-enabled scenes depends on quantity of initial rays being generated, so total required rendering time is increased greatly if both ray branching and DOF are enabled. It may be reasonable to warn users about this if Chunky detects that ray branching and DOF are both enabled when a render is started, or (slightly less reasonable) to disable ray branching if Chunky detects that DOF is enabled.