UI: Add --steam flag

UI/obs-app.cpp

@@ -80,6 +80,7 @@ static string lastLogFile;
 static string lastCrashLogFile;
 
 bool portable_mode = false;
+bool steam = false;
 static bool multi = false;
 static bool log_verbose = false;
 static bool unfiltered_log = false;
@@ -3046,6 +3047,9 @@ int main(int argc, char *argv[])
 				  nullptr)) {
 			opt_disable_missing_files_check = true;
 
+		} else if (arg_is(argv[i], "--steam", nullptr)) {
+			steam = true;
+
 #if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
 		} else if (arg_is(argv[i], "--disable-high-dpi-scaling",
 				  nullptr)) {

UI/obs-app.hpp

@@ -240,6 +240,7 @@ static inline int GetProfilePath(char *path, size_t size, const char *file)
 }
 
 extern bool portable_mode;
+extern bool steam;
 
 extern bool opt_start_streaming;
 extern bool opt_start_recording;