wayland: Add a hint to allow disabling libdecor use

Useful for testing xdg-shell path with compositors like Weston.

include/SDL_hints.h

@@ -1265,6 +1265,17 @@ extern "C" {
  */
 #define SDL_HINT_VIDEO_MINIMIZE_ON_FOCUS_LOSS   "SDL_VIDEO_MINIMIZE_ON_FOCUS_LOSS"
 
+/**
+ *  \brief  A variable controlling whether the libdecor Wayland backend is allowed to be used.
+ *
+ *  This variable can be set to the following values:
+ *    "0"       - libdecor use is disabled.
+ *    "1"       - libdecor use is enabled (default).
+ *
+ *  libdecor is used over xdg-shell when xdg-decoration protocol is unavailable.
+ */
+#define SDL_HINT_VIDEO_WAYLAND_ALLOW_LIBDECOR "SDL_VIDEO_WAYLAND_ALLOW_LIBDECOR"
+
 /**
 *  \brief  A variable that is the address of another SDL_Window* (as a hex string formatted with "%p").
 *  

src/video/wayland/SDL_waylandvideo.c

@@ -37,6 +37,7 @@
 #include "SDL_waylandtouch.h"
 #include "SDL_waylandclipboard.h"
 #include "SDL_waylandvulkan.h"
+#include "SDL_hints.h"
 
 #include <sys/types.h>
 #include <unistd.h>
@@ -555,7 +556,7 @@ Wayland_VideoInit(_THIS)
 
 #ifdef HAVE_LIBDECOR_H
     /* Don't have server-side decorations? Try client-side instead. */
-    if (!data->decoration_manager && SDL_WAYLAND_HAVE_WAYLAND_LIBDECOR) {
+    if (!data->decoration_manager && SDL_WAYLAND_HAVE_WAYLAND_LIBDECOR && SDL_GetHintBoolean(SDL_HINT_VIDEO_WAYLAND_ALLOW_LIBDECOR, SDL_TRUE)) {
         data->shell.libdecor = libdecor_new(data->display, &libdecor_interface);
 
         /* If libdecor works, we don't need xdg-shell anymore. */