Fix issue with moving maximized window in macOS

When opening the Godot editor and maximizing the window by double-clicking the
title bar, users are unable to drag the window with the mouse.

With this commit, `window_set_position` allows the maximized window to be moved
by dragging it. Only the fullscreen window won't be allowed to move.

Fixes godotengine#78758.

(cherry picked from commit d78cb43)

platform/macos/display_server_macos.mm

@@ -3094,7 +3094,7 @@ - (void)popupAction:(id)sender {
 	ERR_FAIL_COND(!windows.has(p_window));
 	WindowData &wd = windows[p_window];
 
-	if (NSEqualRects([wd.window_object frame], [[wd.window_object screen] visibleFrame])) {
+	if (wd.fullscreen) {
 		return;
 	}