Issue #2073 - m-c 523950: Discard decoded frames of very large GIF animations (squashed)

Controlled by image.animated.decode-on-demand.threshold-kb, default 256MB

Includes squashed bugfixes/regressions:
 - m-c 1444537: Shutting down the decode pool should make animated decoders bail early
 - m-c 1628606: Make sure to mark the surface cache entry available before sending the frame complete notification
 - m-c 1502275: Skip recreating the decoder after redecode errors if an animated image is reset
 - m-c 1443232: Don't insert frames into our AnimationFrameBuffer that we consider in error and unusable

image/DecodePool.cpp

@@ -87,6 +87,12 @@ public:
     mMonitor.NotifyAll();
   }
 
+  bool IsShuttingDown() const
+  {
+    MonitorAutoLock lock(mMonitor);
+    return mShuttingDown;
+  }
+
   /// Pushes a new decode work item.
   void PushWork(IDecodingTask* aTask)
   {
@@ -150,7 +156,7 @@ private:
   nsThreadPoolNaming mThreadNaming;
 
   // mMonitor guards the queues and mShuttingDown.
-  Monitor mMonitor;
+  mutable Monitor mMonitor;
   nsTArray<RefPtr<IDecodingTask>> mHighPriorityQueue;
   nsTArray<RefPtr<IDecodingTask>> mLowPriorityQueue;
   bool mShuttingDown;
@@ -299,6 +305,12 @@ DecodePool::Observe(nsISupports*, const char* aTopic, const char16_t*)
   return NS_OK;
 }
 
+bool
+DecodePool::IsShuttingDown() const
+{
+  return mImpl->IsShuttingDown();
+}
+
 void
 DecodePool::AsyncRun(IDecodingTask* aTask)
 {