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/SourceBuffer.h

@@ -187,6 +187,12 @@ public:
   /// @return a count of the bytes in all chunks we've advanced through.
   size_t ByteCount() const { return mByteCount; }
 
+  /// @return the source buffer which owns the iterator.
+  SourceBuffer* Owner() const {
+    MOZ_ASSERT(mOwner);
+    return mOwner;
+  }
+
 private:
   friend class SourceBuffer;