win32: Use STYLE_BORDERLESS when showing a pending fullscreen window

In addition to hiding the border on bordered windows that will immediately become fullscreen, The combination of flags used in STYLE_BORDERLESS_WINDOWED will still show the borders on borderless windows if the initial window size exactly matches the desktop, so STYLE_BORDERLESS must be used instead.

src/video/windows/SDL_windowswindow.c

@@ -1071,16 +1071,16 @@ void WIN_ShowWindow(SDL_VideoDevice *_this, SDL_Window *window)
         WIN_SetWindowPosition(_this, window);
     }
 
-    // If the window isn't borderless and will be fullscreen, use the borderless style to hide the initial borders.
+    /* If the window will immediately become fullscreen, use the borderless style to hide the initial borders.
+     *
+     * Note: The combination of flags used in STYLE_BORDERLESS_WINDOWED will still briefly show the borders if
+     * the initial window size exactly matches the desktop, so STYLE_BORDERLESS must be used instead.
+     */
     if (window->pending_flags & SDL_WINDOW_FULLSCREEN) {
-        if (!(window->flags & SDL_WINDOW_BORDERLESS)) {
-            window->flags |= SDL_WINDOW_BORDERLESS;
-            style = GetWindowLong(hwnd, GWL_STYLE);
-            style &= ~STYLE_MASK;
-            style |= GetWindowStyle(window);
-            SetWindowLong(hwnd, GWL_STYLE, style);
-            window->flags &= ~SDL_WINDOW_BORDERLESS;
-        }
+        style = GetWindowLong(hwnd, GWL_STYLE);
+        style &= ~STYLE_MASK;
+        style |= STYLE_BORDERLESS;
+        SetWindowLong(hwnd, GWL_STYLE, style);
     }
     style = GetWindowLong(hwnd, GWL_EXSTYLE);
     if (style & WS_EX_NOACTIVATE) {