emscriptenaudio: Deal with blocked audio devices better.

Now, if the AudioContext starts in a "suspended" state, because the browser
blocked it from playing by default, we we run the audio "thread" in a timer
and throw away the generated audio. Once the AudioContext is allowed to
resume, we clear this timer.

The end result is that the app will continue to drain its audio queue
instead of consuming more memory over time (and, if it relies on an audio
callback to make progress, continue to run!), with the effect that the
page is merely silent but otherwise functioning as intended.

Once the user interacts with the page and the browser permits the the
AudioContext to run for real, audio should still be in sync, instead of
just starting to play audio that might now be at least several seconds behind.

src/audio/emscripten/SDL_emscriptenaudio.c

@@ -124,6 +124,12 @@ static void EMSCRIPTENAUDIO_CloseDevice(SDL_AudioDevice *device)
                 SDL3.audio.scriptProcessorNode.disconnect();
                 SDL3.audio.scriptProcessorNode = undefined;
             }
+            if (SDL3.audio.silenceTimer !== undefined) {
+                clearInterval(SDL3.audio.silenceTimer);
+            }
+            if (SDL3.audio.silenceBuffer !== undefined) {
+                SDL3.audio.silenceBuffer = undefined
+            }
             SDL3.audio = undefined;
         }
         if ((SDL3.audioContext !== undefined) && (SDL3.audio === undefined) && (SDL3.capture === undefined)) {
@@ -164,7 +170,9 @@ static int EMSCRIPTENAUDIO_OpenDevice(SDL_AudioDevice *device)
                 SDL3.audioContext = new webkitAudioContext();
             }
             if (SDL3.audioContext) {
-                autoResumeAudioContext(SDL3.audioContext);
+                if ((typeof navigator.userActivation) === 'undefined') {  // Firefox doesn't have this (as of August 2023), use autoResumeAudioContext instead.
+                    autoResumeAudioContext(SDL3.audioContext);
+                }
             }
         }
         return SDL3.audioContext === undefined ? -1 : 0;
@@ -219,6 +227,7 @@ static int EMSCRIPTENAUDIO_OpenDevice(SDL_AudioDevice *device)
                 if (SDL3.capture.silenceTimer !== undefined) {
                     clearTimeout(SDL3.capture.silenceTimer);
                     SDL3.capture.silenceTimer = undefined;
+                    SDL3.capture.silenceBuffer = undefined
                 }
                 SDL3.capture.mediaStreamNode = SDL3.audioContext.createMediaStreamSource(stream);
                 SDL3.capture.scriptProcessorNode = SDL3.audioContext.createScriptProcessor($1, $0, 1);
@@ -260,10 +269,37 @@ static int EMSCRIPTENAUDIO_OpenDevice(SDL_AudioDevice *device)
             SDL3.audio.scriptProcessorNode = SDL3.audioContext['createScriptProcessor']($1, 0, $0);
             SDL3.audio.scriptProcessorNode['onaudioprocess'] = function (e) {
                 if ((SDL3 === undefined) || (SDL3.audio === undefined)) { return; }
+                // if we're actually running the node, we don't need the fake callback anymore, so kill it.
+                if (SDL3.audio.silenceTimer !== undefined) {
+                    clearInterval(SDL3.audio.silenceTimer);
+                    SDL3.audio.silenceTimer = undefined;
+                    SDL3.audio.silenceBuffer = undefined;
+                }
                 SDL3.audio.currentOutputBuffer = e['outputBuffer'];
                 dynCall('vi', $2, [$3]);
             };
+
             SDL3.audio.scriptProcessorNode['connect'](SDL3.audioContext['destination']);
+
+            if (SDL3.audioContext.state === 'suspended') {  // uhoh, autoplay is blocked.
+                SDL3.audio.silenceBuffer = SDL3.audioContext.createBuffer($0, $1, SDL3.audioContext.sampleRate);
+                SDL3.audio.silenceBuffer.getChannelData(0).fill(0.0);
+                var silence_callback = function() {
+                    if ((typeof navigator.userActivation) !== 'undefined') {  // Almost everything modern except Firefox (as of August 2023)
+                        if (navigator.userActivation.hasBeenActive) {
+                            SDL3.audioContext.resume();
+                        }
+                    }
+
+                    // the buffer that gets filled here just gets ignored, so the app can make progress
+                    //  and/or avoid flooding audio queues until we can actually play audio.
+                    SDL3.audio.currentOutputBuffer = SDL3.audio.silenceBuffer;
+                    dynCall('vi', $2, [$3]);
+                    SDL3.audio.currentOutputBuffer = undefined;
+                };
+
+                SDL3.audio.silenceTimer = setInterval(silence_callback, ($1 / SDL3.audioContext.sampleRate) * 1000);
+            }
         }, device->spec.channels, device->sample_frames, SDL_OutputAudioThreadIterate, device);
     }