Also, fix a minor bug where when we discard an animated image that is a
full frame animated image, we need to reset
AnimationState::mCompositedFrameInvalid to false, just like we do for
animated images blended by FrameAnimator. This is because it is used as
part of our state checking in FrameAnimator::GetCompositedFrame before
we are willing to yield frame data.
Differential Revision: https://phabricator.services.mozilla.com/D12362