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/launcher.cpp

@@ -594,6 +594,9 @@ int main_implementation(int argc, char *argv[]) {
         GRAPHICS_FORCE_SINGLE_ADAPTER = true;
         GRAPHICS_PREVENT_SECONDARY_WINDOW = true;
     }
+    if (options[launcher::Options::PopnSubMonitorOverride].is_active()) {
+        sysutils::SECOND_MONITOR_OVERRIDE = options[launcher::Options::PopnSubMonitorOverride].value_text();
+    }
     if (options[launcher::Options::LoadMetalGearArcadeModule].value_bool()) {
         attach_mga = true;
     }