popn: boot with 3 monitors (#636)

## Link to GitHub Issue or related Pull Request, if one exists
#618 

## Description of change
Take the monitor hooks added for IIDX/SDVX #612 and apply it to popn as
well now that popn has a subscreen.

There is a debug option in game that actually uses three monitors, which
can be activated via hex edits. This will prevent that from working.

## Testing
Tested with 3 monitors, fs and windowed.

src/spice2x/launcher/options.h

@@ -100,6 +100,7 @@ namespace launcher {
             PopnMusicForceHDMode,
             PopnMusicForceSDMode,
             PopnNoSub,
+            PopnSubMonitorOverride,
             LoadHelloPopnMusicModule,
             LoadGitaDoraModule,
             GitaDoraTwoChannelAudio,