Windows GDK Support (#5830)

* Added GDK

* Simplfied checks in SDL_config_wingdk.h

* Added testgdk sample

* Added GDK readme

* Fixed error in merge of SDL_windows.h

* Additional GDK fixes

* OpenWatcom should not export _SDL_GDKGetTaskQueue

* Formatting fixes

* Moved initialization code into SDL_GDKRunApp

src/main/gdk/SDL_gdk_main.c

@@ -0,0 +1,41 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2022 Sam Lantinga <slouken@libsdl.org>
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+#include "SDL_config.h"
+
+/* Include this so we define UNICODE properly */
+#include "../../core/windows/SDL_windows.h"
+
+/* Include the SDL main definition header */
+#include "SDL.h"
+#include "SDL_main.h"
+
+#ifdef main
+#  undef main
+#endif /* main */
+
+/* This is where execution begins */
+int WINAPI
+WinMain(HINSTANCE hInst, HINSTANCE hPrev, LPSTR szCmdLine, int sw)
+{
+    return SDL_GDKRunApp(SDL_main, NULL);
+}
+
+/* vi: set ts=4 sw=4 expandtab: */