Some fixes for initial window position and size

- X11: set main window position and size hints correctly
- All platforms: update minimum and maximum size of main window at
  startup

Fixes #70984

platform/linuxbsd/x11/display_server_x11.cpp

@@ -4936,6 +4936,11 @@ DisplayServerX11::WindowID DisplayServerX11::_create_window(WindowMode p_mode, V
 		win_rect.position = wpos;
 	}
 
+	// Position and size hints are set from these values before they are updated to the actual
+	// window size, so we need to initialize them here.
+	wd.position = win_rect.position;
+	wd.size = win_rect.size;
+
 	{
 		wd.x11_window = XCreateWindow(x11_display, RootWindow(x11_display, visualInfo.screen), win_rect.position.x, win_rect.position.y, win_rect.size.width > 0 ? win_rect.size.width : 1, win_rect.size.height > 0 ? win_rect.size.height : 1, 0, visualInfo.depth, InputOutput, visualInfo.visual, valuemask, &windowAttributes);
 

scene/main/window.cpp

@@ -1121,6 +1121,7 @@ void Window::_notification(int p_what) {
 						position = DisplayServer::get_singleton()->window_get_position(window_id);
 						size = DisplayServer::get_singleton()->window_get_size(window_id);
 					}
+					_update_window_size(); // Inform DisplayServer of minimum and maximum size.
 					_update_viewport_size(); // Then feed back to the viewport.
 					_update_window_callbacks();
 					RS::get_singleton()->viewport_set_update_mode(get_viewport_rid(), RS::VIEWPORT_UPDATE_WHEN_VISIBLE);