Bug 1287691 (Part 2) - Yield after each frame in the GIF decoder. r=edwin

image/decoders/nsGIFDecoder2.cpp

@@ -475,6 +475,8 @@ nsGIFDecoder2::DoDecode(SourceBufferIterator& aIterator, IResumable* aOnResume)
         return FinishedGlobalColorTable();
       case State::BLOCK_HEADER:
         return ReadBlockHeader(aData);
+      case State::BLOCK_HEADER_AFTER_YIELD:
+        return Transition::To(State::BLOCK_HEADER, BLOCK_HEADER_LEN);
       case State::EXTENSION_HEADER:
         return ReadExtensionHeader(aData);
       case State::GRAPHIC_CONTROL_EXTENSION:
@@ -975,7 +977,7 @@ nsGIFDecoder2::ReadImageDataSubBlock(const char* aData)
   if (subBlockLength == 0) {
     // We hit the block terminator.
     EndImageFrame();
-    return Transition::To(State::BLOCK_HEADER, BLOCK_HEADER_LEN);
+    return Transition::ToAfterYield(State::BLOCK_HEADER_AFTER_YIELD);
   }
 
   if (mGIFStruct.pixels_remaining == 0) {

image/decoders/nsGIFDecoder2.h

@@ -80,6 +80,7 @@ private:
     GLOBAL_COLOR_TABLE,
     FINISHED_GLOBAL_COLOR_TABLE,
     BLOCK_HEADER,
+    BLOCK_HEADER_AFTER_YIELD,
     EXTENSION_HEADER,
     GRAPHIC_CONTROL_EXTENSION,
     APPLICATION_IDENTIFIER,