emscripten: Proxy Emscripten_GetSystemTheme and EMSCRIPTENAUDIO_OpenDevice to the main thread.

(cherry picked from commit 168d1a9253)

src/audio/emscripten/SDL_emscriptenaudio.c

@@ -189,7 +189,7 @@ static bool EMSCRIPTENAUDIO_OpenDevice(SDL_AudioDevice *device)
     }
 
     // limit to native freq
-    device->spec.freq = EM_ASM_INT({ return Module['SDL3'].audioContext.sampleRate; });
+    device->spec.freq = MAIN_THREAD_EM_ASM_INT({ return Module['SDL3'].audioContext.sampleRate; });
     device->sample_frames = SDL_GetDefaultSampleFramesFromFreq(device->spec.freq) * 2;  // double the buffer size, some browsers need more, and we'll just have to live with the latency.
 
     SDL_UpdatedAudioDeviceFormat(device);

src/video/emscripten/SDL_emscriptenvideo.c

@@ -64,7 +64,7 @@ static SDL_SystemTheme Emscripten_GetSystemTheme(void)
     /* Technically, light theme can mean explicit light theme or no preference.
        https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-color-scheme#syntax */
 
-    int theme_code = EM_ASM_INT({
+    int theme_code = MAIN_THREAD_EM_ASM_INT({
         if (!window.matchMedia) {
             return -1;
         }