Ignore thread models when compiling with NO_THREAD

The thread model option for physics (2D) and rendering (single-unsafe,
single-safe, multithread), was causing crashes/locks when set as
multithreaded and exported for a platform that does not support threads
(namely HTML5).

This commit ensures that when threads support is not available, that
option is ignored, and the equivalent of "single-unsafe" is always used
instead.

main/main.cpp

@@ -1274,9 +1274,13 @@ Error Main::setup(const char *execpath, int argc, char *argv[], bool p_second_ph
 	}
 
 	if (rtm >= 0 && rtm < 3) {
+#ifdef NO_THREADS
+		rtm = OS::RENDER_THREAD_UNSAFE; // No threads available on this platform.
+#else
 		if (editor) {
 			rtm = OS::RENDER_THREAD_SAFE;
 		}
+#endif
 		OS::get_singleton()->_render_thread_mode = OS::RenderThreadMode(rtm);
 	}