Support DisposalMethod::kRestorePrevious in MultiFrameCodec and fix the apng problem.

lib/ui/painting/image_generator_apng.cc

@@ -536,6 +536,13 @@ bool APNGImageGenerator::DemuxNextImageInternal() {
           SkCodecAnimation::DisposalMethod::kKeep) {
     // Mark the required frame as the previous frame in all cases.
     image->frame_info->required_frame = images_.size() - 1;
+  } else if (images_.size() > (first_frame_index_ + 1) &&
+             images_.back().frame_info->disposal_method ==
+                 SkCodecAnimation::DisposalMethod::kRestorePrevious) {
+    // Mark the required frame as the last previous frame
+    // It is not valid if there are 2 or above frames set |disposal_method| to
+    // |kRestorePrevious|. But it also works in MultiFrameCodec.
+    image->frame_info->required_frame = images_.size() - 2;
   }
 
   // Calling SkCodec::getInfo at least once prior to decoding is mandatory.

lib/ui/painting/multi_frame_codec.cc

@@ -116,9 +116,9 @@ MultiFrameCodec::State::GetNextFrameImage(
   std::optional<unsigned int> prior_frame_index = std::nullopt;
 
   if (requiredFrameIndex != SkCodec::kNoFrame) {
-    // We currently assume that frames can only ever depend on the immediately
-    // previous frame, if any. This means that
-    // `DisposalMethod::kRestorePrevious` is not supported.
+    // We are here when the frame said |disposal_method| is
+    // `DisposalMethod::kKeep` or `DisposalMethod::kRestorePrevious` and
+    // |requiredFrameIndex| is set to ex-frame or ex-ex-frame.
     if (lastRequiredFrame_ == nullptr) {
       FML_DLOG(INFO)
           << "Frame " << nextFrameIndex_ << " depends on frame "
@@ -146,9 +146,14 @@ MultiFrameCodec::State::GetNextFrameImage(
     return std::make_pair(nullptr, decode_error);
   }
 
-  // Hold onto this if we need it to decode future frames.
-  if (frameInfo.disposal_method == SkCodecAnimation::DisposalMethod::kKeep ||
-      lastRequiredFrame_) {
+  // Hold onto this if we need it to decode future frames except that current
+  // frame's disposal_method is kRestorePrevious.
+  const bool dont_keep_current_frame =
+      frameInfo.disposal_method ==
+      SkCodecAnimation::DisposalMethod::kRestorePrevious;
+  if (!dont_keep_current_frame &&
+      (frameInfo.disposal_method == SkCodecAnimation::DisposalMethod::kKeep ||
+       lastRequiredFrame_)) {
     lastRequiredFrame_ = std::make_unique<SkBitmap>(bitmap);
     lastRequiredFrameIndex_ = nextFrameIndex_;
   }