make dispmanx layer user-configurable

[matiaspl]

Adding a command line option for choosing the layer number would be great :)

[hex007]

#1 was meant to do that but was abandoned. I will see if I can add that option

[matiaspl]

You have layer defined in https://github.com/hex007/eop/blob/master/eop.c#L64 and the command line parsing code just a few lines below, so it's perfectly doable 👍

[nagualcode]

#4