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/decoders/nsWebPDecoder.h

@@ -21,6 +21,8 @@ class nsWebPDecoder final : public Decoder
 public:
   virtual ~nsWebPDecoder();
 
+  DecoderType GetType() const override { return DecoderType::WEBP; }
+
 protected:
   LexerResult DoDecode(SourceBufferIterator& aIterator,
                        IResumable* aOnResume) override;