emscripten: Automatically resume audio contexts

This uses the mechanism added in emscripten-core/emscripten#10843
which was applied to SDL1 and OpenAL. This adds the same for SDL2.

This also reverts commit 865eaddffed50dbd13e6564c3f73902472cf74e8
which did something similar, but the new mechanism is more effective.

src/audio/emscripten/SDL_emscriptenaudio.c

@@ -215,6 +215,9 @@ EMSCRIPTENAUDIO_OpenDevice(_THIS, void *handle, const char *devname, int iscaptu
             } else if (typeof(webkitAudioContext) !== 'undefined') {
                 SDL2.audioContext = new webkitAudioContext();
             }
+            if (SDL2.audioContext) {
+                autoResumeAudioContext(SDL2.audioContext);
+            }
         }
         return SDL2.audioContext === undefined ? -1 : 0;
     }, iscapture);