refactor(templates,tests): make sync an extern instead of parameter to render

This is to be able to use CreateThread with su_render_song; let's worry about multicore rendering sometime in the future.

tests/test_renderer.c

@@ -41,11 +41,7 @@ int main(int argc, char* argv[]) {
     su_load_gmdls();
     #endif
 
-    #ifdef SU_SYNC
-    su_render_song(buf, syncBuf);
-    #else
     su_render_song(buf);
-    #endif
 
 #if defined (_WIN32)
     CreateDirectory(actual_output_folder, NULL);