gtk: "pixel-aspect-ratio" change not effective after creation

Changing the "pixel-aspect-ratio" property doesn't seem to have any effects after its initial (default) setting, as _calculate_par() is never called again.

This could be important to implement support for screens with non-square pixels, as totem supports: https://gitlab.gnome.org/GNOME/totem/-/blob/gnome-3-38/src/backend/bacon-video-widget.c#L422-467

I'm not certain it's a very important feature, but it also begs the question of what this property if useful for if it doesn't work as expected...

I have a very crude patch that schedules a _queue_draw idle callback when the property changes, which seems to change the ratio as expected.