track which shutter(s) have been actually used in a given exposure.

Given the current configuration, sps need to figure out which shutters are required to achieve the exposure and pass down this information to the enuActor.

So far, it has been mostly both or red.
But we also have seen blue only, if there is no red shutter for instance.

Just do it right, so it's tracked in the header and would no longer need any hack.