GeezerVison

When a display shows up under a new prefs id (after wake or re-docking, for example), setupCurrentAndMaxValues converts the DDC value it just read before maxDDC is stored. With min and max both 0, convDDCToValue divides 0 by 0 and the NaN gets saved as the current value. The next brightness change then traps on UInt16(NaN) in convValueToDDC; the min/max clamps don't help, as they pass NaN through.

This PR stores maxDDC before converting the read value. A display that reports a maximum of 0 falls back to DDC_MAX_DETECT_LIMIT (I've seen an iiyama do this for volume over HDMI), and a NaN that earlier versions already saved is removed, so the value is read again.

I reproduced it on 4.4.0 by removing the maxDDC keys from the prefs: every command logged Current internal value: nan and the next brightness key press crashed. With this change the same steps give valid values. Probably the same crash as #1919. #1874 also guards against min == max in the conversions, but doesn't cover the ordering or the NaN values already in people's prefs.