Add `SDLOpaquePointerArray` and use it for SDL_GetWindows()

`SDLPointerArray` is inadequate as it does `*(SDL_Window*)ptr` which doesn't make sense
as SDL_Window* is an opaque pointer type

SDL3-CS/SDL3/SDL_video.cs

@@ -107,11 +107,11 @@ namespace SDL
         }
 
         [MustDisposeResource]
-        public static unsafe SDLPointerArray<SDL_Window>? SDL_GetWindows()
+        public static unsafe SDLOpaquePointerArray<SDL_Window>? SDL_GetWindows()
         {
             int count;
             var array = SDL_GetWindows(&count);
-            return SDLArray.Create(array, count);
+            return SDLArray.CreateOpaque(array, count);
         }
     }
 }

SDL3-CS/SDLArray.cs

@@ -63,5 +63,15 @@ namespace SDL
 
             return new SDLPointerArray<T>(array, count);
         }
+
+        [MustDisposeResource]
+        internal static SDLOpaquePointerArray<T>? CreateOpaque<T>(T** array, int count)
+            where T : unmanaged
+        {
+            if (array == null)
+                return null;
+
+            return new SDLOpaquePointerArray<T>(array, count);
+        }
     }
 }

SDL3-CS/SDLOpaquePointerArray.cs

@@ -0,0 +1,46 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using System.Diagnostics;
+using JetBrains.Annotations;
+
+namespace SDL
+{
+    [MustDisposeResource]
+    public sealed unsafe class SDLOpaquePointerArray<T> : IDisposable
+        where T : unmanaged
+    {
+        private readonly T** array;
+        public readonly int Count;
+        private bool isDisposed;
+
+        internal SDLOpaquePointerArray(T** array, int count)
+        {
+            this.array = array;
+            Count = count;
+        }
+
+        public T* this[int index]
+        {
+            get
+            {
+                ObjectDisposedException.ThrowIf(isDisposed, this);
+                ArgumentOutOfRangeException.ThrowIfNegative(index);
+                ArgumentOutOfRangeException.ThrowIfGreaterThanOrEqual(index, Count);
+                Debug.Assert(array[index] != null);
+                return array[index];
+            }
+        }
+
+        public void Dispose()
+        {
+            if (isDisposed)
+                return;
+
+            isDisposed = true;
+
+            SDL3.SDL_free(array);
+        }
+    }
+}