Logic for choosing default display-mode is slightly wrong

[lurch]

Found this while investigating #142 but the issues are definitely different (but perhaps the issue below is an influence on that issue?)...

Try this:

1. Load NOOBS v1.3.2 onto a fresh SD card, and load it up on a Pi
2. Wait until the list of OSes appears
3. Don't install any OS, but press '2' on the keyboard to switch to HDMI safe mode
4. The screen goes low-res
5. Say 'Yes' to making it permanent
6. Power off the Pi, and power it on again
7. NOOBS loads up in low-res mode (as expected)
8. Power off the Pi, and power it on again
9. NOOBS boots up in high-res mode again (not expected!!)

I believe this occurs as a combination of https://github.com/raspberrypi/noobs/blob/master/recovery/main.cpp#L91 and https://github.com/raspberrypi/noobs/blob/master/recovery/main.cpp#L125 and https://github.com/raspberrypi/noobs/blob/master/recovery/mainwindow.cpp#L172 - as far as NOOBS is concerned, there's absolutely no difference between no 'display' option being specified in recovery.cmdline, and the display=1 option being specified. But I believe there needs to be a distinction made (perhaps defaultDisplay in main.cpp should be initialised to "-1" ?), so that NOOBS can tell the difference between "no display option specified, so stick to the previous default" and "a display option was specified, so override the default".

[Ruffio]

@lurch is this still relevant?

[lurch]

I've not tested this specific scenario for a while, but I've not made any changes to this area of NOOBS, so yes I suspect it's still relevant.
The lack of any other comments would seem to indicate it's low-priority though.