LibVideo: Add a forwarding header

And change some non-plain structs to classes for consistency.

Userland/Libraries/LibVideo/DecoderError.h

@@ -10,12 +10,11 @@
 #include <AK/Error.h>
 #include <AK/Format.h>
 #include <AK/SourceLocation.h>
+#include <LibVideo/Forward.h>
 #include <errno.h>
 
 namespace Video {
 
-struct DecoderError;
-
 template<typename T>
 using DecoderErrorOr = ErrorOr<T, DecoderError>;
 
@@ -33,7 +32,7 @@ enum class DecoderErrorCategory : u32 {
     NotImplemented,
 };
 
-struct DecoderError {
+class DecoderError {
 public:
     static DecoderError with_description(DecoderErrorCategory category, StringView description)
     {

Userland/Libraries/LibVideo/Forward.h

@@ -0,0 +1,19 @@
+/*
+ * Copyright (c) 2023, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+namespace Video {
+
+class DecoderError;
+class FrameQueueItem;
+class PlaybackManager;
+class Sample;
+class Track;
+class VideoDecoder;
+class VideoFrame;
+
+}

Userland/Libraries/LibVideo/PlaybackManager.h

@@ -23,7 +23,8 @@
 
 namespace Video {
 
-struct FrameQueueItem {
+class FrameQueueItem {
+public:
     static constexpr Time no_timestamp = Time::min();
 
     enum class Type {

Userland/Libraries/LibVideo/Track.h

@@ -20,7 +20,7 @@ enum class TrackType : u32 {
     Subtitles,
 };
 
-struct Track {
+class Track {
     struct VideoData {
         Time duration {};
         u64 pixel_width { 0 };