changed fading complete screen only if all viewports have changed

(... or if the global border has changed, of course)

This change partially reverses the behaviour caused by commit 5c6c42f1
("fixed fading complete screen if playfield viewport has changed"),
which prevented partially fading screens that only partially changed.

The main intention of this change is to make it possible that the tape
recorder viewport, if unchanged, keeps being visible while fading the
remaining parts of the screen (especially with artwork sets without
global border graphics).

However, if *all* viewports changed, the whole window will be faded.