iidx,sdvx: disable subscreen overlay if -monitor is in use (#471)

## Link to GitHub Issue, if one exists
#345 

## Description of change
If TDJ / VM mode is active AND user is enabled `-monitor` option (and
the value isn't 0, the primary monitor)

then disable the subscreen overlay, show an error message instead.

We're doing this because both games cannot accept emulated touch input
if `-monitor` option is in use. There is not much point in displaying
the second screen's image, other than to REALLY confuse people when
nothing happens when they click on it.

## Testing
Tested both games with `-monitor`.

src/spice2x/games/iidx/iidx.h

@@ -33,6 +33,7 @@ namespace games::iidx {
     extern char IIDXIO_LED_TICKER[10];
     extern bool IIDXIO_LED_TICKER_READONLY;
     extern std::mutex IIDX_LED_TICKER_LOCK;
+    extern bool IIDX_TDJ_MONITOR_WARNING;
 
     constexpr int IIDX_TAPELED_TOTAL = 17;
     // data mapping