video: Add SDL_SetWindowMouseRect.

This API and implementation comes from the Unreal Engine branch of SDL, which
originally called this "SDL_ConfineCursor".

Some minor cleanup and changes for consistency with the rest of SDL_video, but
there are two major changes:

1. The coordinate system has been changed so that `rect` is _window_ relative
   and not _screen_ relative, making it easier to implement without having
   global access to the display.
2. The UE version unset all rects when passing `NULL` as a parameter for
   `window`, this has been removed as it was an unused feature anyhow.

Currently this is only implemented for X, but can be supported on Wayland and
Windows at minimum too.

include/SDL_video.h

@@ -1497,6 +1497,23 @@ extern DECLSPEC int SDLCALL SDL_SetWindowModalFor(SDL_Window * modal_window, SDL
  */
 extern DECLSPEC int SDLCALL SDL_SetWindowInputFocus(SDL_Window * window);
 
+/**
+ * Confines the cursor in the specified rect area of a window.
+ *
+ * Note that this does NOT grab the cursor, it only defines the area a cursor
+ * is restricted to when the window has mouse focus.
+ *
+ * \param window The window that will be associated with the barrier.
+ * \param rect A rectangle area in window-relative coordinates. If NULL the
+ *  barrier for the specified window will be destroyed.
+ *
+ * \returns 0 on success or a negative error code on failure; call
+ *          SDL_GetError() for more information.
+ *
+ * \sa SDL_SetWindowGrab
+ */
+extern DECLSPEC int SDLCALL SDL_SetWindowMouseRect(SDL_Window * window, const SDL_Rect * rect);
+
 /**
  * Set the gamma ramp for the display that owns a given window.
  *