iidx: turn off camera access by default (#428)

## Link to GitHub Issue, if one exists
For #345 
Similar idea as https://github.com/spice2x/spice2x.github.io/pull/350

## Description of change
Disable camera access by default by setting `CONNECT_CAMERA` environment
var to 0 before launching. This was one of those "required" things
people clicked on, but now it's done automatically to reduce friction
for new users.

On cab setups (i.e., running without -iidx module), cameras will remain
on, unless user overrides it.

On franken setups (partial I/O) - user will need to specify what they
want via the new option, we can't guess correctly.

## Testing

src/spice2x/games/iidx/iidx.h

@@ -14,7 +14,7 @@ namespace games::iidx {
     // settings
 
     extern bool FLIP_CAMS;
-    extern bool DISABLE_CAMS;
+    extern std::optional<bool> DISABLE_CAMS;
     extern bool TDJ_CAMERA;
     extern bool TDJ_CAMERA_PREFER_16_9;
     extern std::optional<std::string> TDJ_CAMERA_OVERRIDE;