Wayland: Add SDL_HINT_VIDEO_WAYLAND_PREFER_LIBDECOR

This hint allows libdecor to be used even when xdg-decoration is
available. It's mostly useful for debugging libdecor, but could in
theory be used by applications which want to (for example) bundle their
own libdecor plugins.

include/SDL_hints.h

@@ -1475,6 +1475,20 @@ extern "C" {
  */
 #define SDL_HINT_VIDEO_WAYLAND_ALLOW_LIBDECOR "SDL_VIDEO_WAYLAND_ALLOW_LIBDECOR"
 
+/**
+ *  \brief  A variable controlling whether the libdecor Wayland backend is preferred over native decrations.
+ *
+ *  When this hint is set, libdecor will be used to provide window decorations, even if xdg-decoration is
+ *  available. (Note that, by default, libdecor will use xdg-decoration itself if available).
+ *
+ *  This variable can be set to the following values:
+ *    "0"       - libdecor is enabled only if server-side decorations are unavailable.
+ *    "1"       - libdecor is always enabled if available.
+ *
+ *  libdecor is used over xdg-shell when xdg-decoration protocol is unavailable.
+ */
+#define SDL_HINT_VIDEO_WAYLAND_PREFER_LIBDECOR "SDL_VIDEO_WAYLAND_PREFER_LIBDECOR"
+
 /**
 *  \brief  A variable that is the address of another SDL_Window* (as a hex string formatted with "%p").
 *