[mythtv] GUI size for playback, only when zooming

[Preston Crow]

I've been trying to add an option to use the GUI size for playback when
using zoom modes.  For normal shows, the overscan doesn't bother me, but
when zooming a letterboxed show, the overscan is way too much.

Unfortunately, it seems in the code that the code that checks
GuiSizeForTV is only called when the playback is initialized, not when
the zoom mode is changed, so my first approach of having it check if
either that option is set or we're in a zoom mode doesn't work.

Any thoughts on how I should implement this?