OpenDMX/FTDI DMX Input

[peternewman]

We currently only support DMX out from the FTDI based interfaces. It would be nice to also support DMX in.

Example code from Enttec here (unclear about licensing, although frontpage says GPLv2):
http://prdownloads.sourceforge.net/opendmxusb/bc5_dmxusb_soft_141.zip?download

[peternewman]

@Keeper-of-the-Keys has RDM working in #1541 so DMX input only should be a fairly easy subset of this.

[Keeper-of-the-Keys]

I'm not sure how easy this will be, detecting the beginning and end of a DMX frame (and properly aligning to it) is though on FTDI/libusb as far as I can tell, but I definitely want to try at some later point.