iidx: show error message for emulated I/O mismatch in LDJ/TDJ sub overlay (#555)

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

## Description of change
When DLL is configured to use TDJ I/O but spice is in LDJ mode, show an
error in LED ticker suggesting the user to turn on `-iidxtdj`.

When DLL is configured to use LDJ I/O but spice is in TDJ mode, show an
error in TDJ overaly suggesting the user to fix the DLL.

Fix a small bug that caused a white window to be created when `-w` is
enabled even when `-iidxtdj` is not, but DLL is using TDJ I/O.

src/spice2x/overlay/windows/generic_sub.h

@@ -31,6 +31,7 @@ namespace overlay::windows {
 
     protected:
         virtual void touch_transform(const ImVec2 xy_in, LONG *x_out, LONG *y_out);
+        virtual void check_for_errors();
         ImVec2 overlay_content_top_left;
         ImVec2 overlay_content_size;
         std::optional<std::string> disabled_message = std::nullopt;