Null video driver: Add driver parameter to run until exit

src/video/null_v.h

@@ -17,7 +17,8 @@
 /** The null video driver. */
 class VideoDriver_Null : public VideoDriver {
 private:
-	uint ticks; ///< Amount of ticks to run.
+	int ticks; ///< Amount of ticks to run.
+	bool until_exit;
 
 public:
 	const char *Start(const char * const *param) override;