LibVideo: Allow the VP9 decoder to queue multiple frames

Frames will now be queued for retrieval by the user of the decoder.
When the end of the current queue is reached, a DecoderError of
category NeedsMoreInput will be emitted, allowing the caller to react
by displaying what was previously retrieved for sending more samples.

Userland/Libraries/LibVideo/DecoderError.h

@@ -22,6 +22,7 @@ using DecoderErrorOr = ErrorOr<T, DecoderError>;
 enum class DecoderErrorCategory : u32 {
     Unknown,
     IO,
+    NeedsMoreInput,
     EndOfStream,
     Memory,
     // The input is corrupted.