Decode using the last cached required frame (#7715)

`MultiFrameCodec` now uses whatever previously cached required frame is
available instead of panicking if it doesn't have the exact frame
requested by `SkCodec::FrameInfo#fRequiredFrame`.

`SkCodec::FrameInfo#fRequiredFrame` doesn't point to the one and only
frame that's required to decode the given frame. It points to the latest
frame that's disposal method none and filling a greater surface area
than the current frame. The last required frame `MultiFrameCodec` has
actually cached is also valid in these cases and can be supplied as
`fPriorFrame` instead. [flutter/flutter#26757](https://github.com/flutter/flutter/issues/26757#issuecomment-459522530)
has a more detailed explanation.

Fixes flutter/flutter#26757

lib/ui/painting/codec.cc

@@ -408,11 +408,15 @@ sk_sp<SkImage> MultiFrameCodec::GetNextFrameImage(
     options.fFrameIndex = nextFrameIndex_;
     const int requiredFrameIndex = frameInfos_[nextFrameIndex_].fRequiredFrame;
     if (requiredFrameIndex != SkCodec::kNoFrame) {
-      if (lastRequiredFrame_ == nullptr ||
-          lastRequiredFrameIndex_ != requiredFrameIndex) {
+      if (lastRequiredFrame_ == nullptr) {
         FML_LOG(ERROR) << "Frame " << nextFrameIndex_ << " depends on frame "
-                       << requiredFrameIndex << " which has not been cached.";
+                       << requiredFrameIndex
+                       << " and no required frames are cached.";
         return NULL;
+      } else if (lastRequiredFrameIndex_ != requiredFrameIndex) {
+        FML_DLOG(INFO) << "Required frame " << requiredFrameIndex
+                       << " is not cached. Using " << lastRequiredFrameIndex_
+                       << " instead";
       }
 
       if (lastRequiredFrame_->getPixels() &&