For animations, you often want an animated render seed (noise pattern).
This could be done by e.g. setting a driver on the seed value.
Now it's a little checkbox, that can be enabled.
The animated seed is based on the current Blender frame and
the seed value itself. Simply enabling it, will already result in an animated
seed (different on each Blender frame), but it can be randomized further
by setting a different seed value.
Disabled per default, so no backward compatibility break.
Differential Revision: https://developer.blender.org/D1285
7c06190882