DesktopGL implementation for MessageBox.Show() (MonoGame#8720)

Here's an implementation of ```MessageBox.Show()``` for DesktopGL.

All platforms should be supported now.

Author: ThomasFOG

MonoGame.Framework/MonoGame.Framework.DesktopGL.csproj

@@ -74,7 +74,7 @@
     <Compile Include="Platform\Input\KeysHelper.cs" />
     <Compile Include="Platform\Input\Mouse.SDL.cs" />
     <Compile Include="Platform\Input\MouseCursor.SDL.cs" />
-    <Compile Include="Platform\Input\MessageBox.Default.cs" />
+    <Compile Include="Platform\Input\MessageBox.SDL.cs" />
     <Compile Include="Platform\Graphics\GraphicsContext.SDL.cs" />
     <Compile Include="Platform\Graphics\OpenGL.SDL.cs" />
     <Compile Include="Platform\Graphics\WindowInfo.SDL.cs" />

MonoGame.Framework/Platform/Input/MessageBox.SDL.cs

@@ -0,0 +1,87 @@
+using MonoGame.Framework.Utilities;
+using System;
+using System.Collections.Generic;
+using System.Runtime.InteropServices;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Microsoft.Xna.Framework.Input
+{
+    public static partial class MessageBox
+    {
+        private static TaskCompletionSource<int?> _tcs;
+
+        private static Task<int?> PlatformShow(string title, string description, List<string> buttons)
+        {
+            _tcs = new TaskCompletionSource<int?>();
+
+            Sdl.Window.MessageBoxData data = new Sdl.Window.MessageBoxData();
+            data.flags = 0;
+            data.title = title;
+            data.message = description;
+            data.numbuttons = 0;
+            data.buttons = IntPtr.Zero;
+            data.colorScheme = IntPtr.Zero;
+
+            if (buttons != null &&
+                buttons.Count > 0)
+            {
+                Sdl.Window.MessageBoxButtonData[] buttonData = new Sdl.Window.MessageBoxButtonData[buttons.Count];
+
+                for (int i = 0; i < buttons.Count; i++)
+                {
+                    // We need to reverse button order on Windows only
+                    // SDL3 has a flag to reverse that but SDL2 doesn't
+                    int reverseIndex = (CurrentPlatform.OS == OS.Windows ? buttons.Count - i - 1 : i);
+
+                    buttonData[i] = new Sdl.Window.MessageBoxButtonData();
+                    buttonData[i].flags = 0;
+                    buttonData[i].buttonid = reverseIndex;
+                    buttonData[i].text = IntPtr.Zero;
+
+                    if (buttons[i] != null)
+                    {
+                        // convert to C string pointer data because we need to marshal a struct array
+                        byte[] bytes = Encoding.UTF8.GetBytes(buttons[reverseIndex]);
+                        IntPtr mem = Marshal.AllocHGlobal(bytes.Length + 1);
+                        Marshal.Copy(bytes, 0, mem, bytes.Length);
+                        unsafe
+                        {
+                            ((byte*)mem)[bytes.Length] = 0;
+                        }
+
+                        buttonData[i].text = mem;
+                    }
+                }
+
+                data.numbuttons = buttons.Count;
+
+                // convert the struct array to pointer
+                unsafe
+                {
+                    fixed (Sdl.Window.MessageBoxButtonData* buttonsPtr = &buttonData[0])
+                    {
+                        data.buttons = (IntPtr)buttonsPtr;
+                    }
+                }
+            }
+
+            int result = -1;
+
+            int error = Sdl.Window.ShowMessageBox(ref data, out result);
+
+            if (error == 0)
+                _tcs.SetResult(result);
+            else
+                _tcs.SetResult(-1);
+
+            return _tcs.Task;
+        }
+
+        private static void PlatformCancel(int? result)
+        {
+            if (_tcs != null)
+                _tcs.SetResult(result);
+        }
+    }
+}

MonoGame.Framework/Platform/SDL/SDL2.cs

@@ -491,6 +491,30 @@ public static void SetTitle(IntPtr handle, string title)
         [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
         public delegate int d_sdl_getwindowborderssize(IntPtr window, out int top, out int left, out int right, out int bottom);
         public static d_sdl_getwindowborderssize GetBorderSize = FuncLoader.LoadFunction<d_sdl_getwindowborderssize>(NativeLibrary, "SDL_GetWindowBordersSize");
+
+        [StructLayout(LayoutKind.Sequential)]
+        public struct MessageBoxData
+        {
+            public uint flags;
+            public IntPtr window;
+            public string title;
+            public string message;
+            public int numbuttons;
+            public IntPtr buttons;
+            public IntPtr colorScheme;
+        }
+
+        [StructLayout(LayoutKind.Sequential)]
+        public struct MessageBoxButtonData
+        {
+            public uint flags;
+            public int buttonid;
+            public IntPtr text;
+        }
+
+        [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
+        public delegate int d_sdl_showmessagebox(ref MessageBoxData messageboxdata, out int buttonid);
+        public static d_sdl_showmessagebox ShowMessageBox = FuncLoader.LoadFunction<d_sdl_showmessagebox>(NativeLibrary, "SDL_ShowMessageBox");
     }
 
     public static class Display