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.

(cherry picked from commit f3c6ac1d7183a9aea2809ee283a5cd899caf9f3d)

main/main.cpp

@@ -1118,9 +1118,13 @@ 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);
 	}
 

servers/physics_2d/physics_2d_server_sw.cpp

@@ -1455,7 +1455,11 @@ Physics2DServerSW::Physics2DServerSW() {
 	island_count = 0;
 	active_objects = 0;
 	collision_pairs = 0;
+#ifdef NO_THREADS
+	using_threads = false;
+#else
 	using_threads = int(ProjectSettings::get_singleton()->get("physics/2d/thread_model")) == 2;
+#endif
 	flushing_queries = false;
 };
 

servers/physics_2d/physics_2d_server_wrap_mt.h

@@ -326,6 +326,9 @@ public:
 	template <class T>
 	static Physics2DServer *init_server() {
 
+#ifdef NO_THREADS
+		return memnew(T); // Always single unsafe when no threads are available.
+#else
 		int tm = GLOBAL_DEF("physics/2d/thread_model", 1);
 		if (tm == 0) // single unsafe
 			return memnew(T);
@@ -333,6 +336,7 @@ public:
 			return memnew(Physics2DServerWrapMT(memnew(T), false));
 		else // multi threaded
 			return memnew(Physics2DServerWrapMT(memnew(T), true));
+#endif
 	}
 
 #undef ServerNameWrapMT