[rcore] `GetWindowPosition()` possibly incorrect after virtual screen space rearrangement

When a user rearranges their monitors in the virtual screen space, or a monitor is disconnected, that may implicitly change window positions, despite the windows remaining stationary. This is not reflected by Raylib's GetWindowPosition function, which uses its own position buffering that doesn't get updated.

With the GLFW backend, it may be possible to set glfwSetMonitorCallback to get notified by such changes.