Add support for modal windows to more platforms

- Adds support for modal windows to Win32, Mac, and Haiku, and enhances functionality on Wayland and X11, which previous set only the parent window, but not the modal state.
- Windows can be declared modal at creation time, and the modal state can be toggled at any time via SDL_SetWindowModalFor() (tested with UE5 through sdl2-compat).
- Allows dynamic unparenting/reparenting of windows.
- Includes a modal window test.

include/SDL3/SDL_video.h

@@ -144,6 +144,7 @@ typedef Uint32 SDL_WindowFlags;
 #define SDL_WINDOW_INPUT_FOCUS          0x00000200U /**< window has input focus */
 #define SDL_WINDOW_MOUSE_FOCUS          0x00000400U /**< window has mouse focus */
 #define SDL_WINDOW_EXTERNAL             0x00000800U /**< window not created by SDL */
+#define SDL_WINDOW_MODAL                0x00001000U /**< window is modal */
 #define SDL_WINDOW_HIGH_PIXEL_DENSITY   0x00002000U /**< window uses high pixel density back buffer if possible */
 #define SDL_WINDOW_MOUSE_CAPTURE        0x00004000U /**< window has mouse captured (unrelated to MOUSE_GRABBED) */
 #define SDL_WINDOW_ALWAYS_ON_TOP        0x00008000U /**< window should always be above others */
@@ -907,13 +908,15 @@ extern DECLSPEC SDL_Window *SDLCALL SDL_CreatePopupWindow(SDL_Window *parent, in
  *   with Metal rendering
  * - `SDL_PROP_WINDOW_CREATE_MINIMIZED_BOOLEAN`: true if the window should
  *   start minimized
+ * - `SDL_PROP_WINDOW_CREATE_MODAL_BOOLEAN`: true if the window is modal to its
+ *   parent
  * - `SDL_PROP_WINDOW_CREATE_MOUSE_GRABBED_BOOLEAN`: true if the window starts
  *   with grabbed mouse focus
  * - `SDL_PROP_WINDOW_CREATE_OPENGL_BOOLEAN`: true if the window will be used
  *   with OpenGL rendering
  * - `SDL_PROP_WINDOW_CREATE_PARENT_POINTER`: an SDL_Window that will be the
- *   parent of this window, required for windows with the "toolip" and "menu"
- *   properties
+ *   parent of this window, required for windows with the "toolip", "menu", and
+ *   "modal" properties
  * - `SDL_PROP_WINDOW_CREATE_RESIZABLE_BOOLEAN`: true if the window should be
  *   resizable
  * - `SDL_PROP_WINDOW_CREATE_TITLE_STRING`: the title of the window, in UTF-8
@@ -1008,6 +1011,7 @@ extern DECLSPEC SDL_Window *SDLCALL SDL_CreateWindowWithProperties(SDL_Propertie
 #define SDL_PROP_WINDOW_CREATE_MENU_BOOLEAN                        "menu"
 #define SDL_PROP_WINDOW_CREATE_METAL_BOOLEAN                       "metal"
 #define SDL_PROP_WINDOW_CREATE_MINIMIZED_BOOLEAN                   "minimized"
+#define SDL_PROP_WINDOW_CREATE_MODAL_BOOLEAN                       "modal"
 #define SDL_PROP_WINDOW_CREATE_MOUSE_GRABBED_BOOLEAN               "mouse_grabbed"
 #define SDL_PROP_WINDOW_CREATE_OPENGL_BOOLEAN                      "opengl"
 #define SDL_PROP_WINDOW_CREATE_PARENT_POINTER                      "parent"
@@ -2000,7 +2004,12 @@ extern DECLSPEC int SDLCALL SDL_SetWindowOpacity(SDL_Window *window, float opaci
 extern DECLSPEC int SDLCALL SDL_GetWindowOpacity(SDL_Window *window, float *out_opacity);
 
 /**
- * Set the window as a modal for another window.
+ * Set the window as a modal to a parent window.
+ *
+ * If the window is already modal to an existing window, it will be reparented to the new owner.
+ * Setting the parent window to null unparents the modal window and removes modal status.
+ *
+ * Setting a window as modal to a parent that is a descendent of the modal window results in undefined behavior.
  *
  * \param modal_window the window that should be set modal
  * \param parent_window the parent window for the modal window
@@ -2181,6 +2190,8 @@ extern DECLSPEC int SDLCALL SDL_FlashWindow(SDL_Window *window, SDL_FlashOperati
 /**
  * Destroy a window.
  *
+ * Any popups or modal windows owned by the window will be recursively destroyed as well.
+ *
  * If `window` is NULL, this function will return immediately after setting
  * the SDL error message to "Invalid window". See SDL_GetError().
  *