added always redrawing global border when redrawing playfield area

Before, the global border was only redrawn if its graphics had changed
or if any of the viewport areas (playfield or doors) had changed their
position or size.

Now, it is always redrawn if the playfield area is cleared (to redraw
its content). This has the advantage that accidentally damaged global
border areas (caused by buggy graphics definitions from custom artwork
sets) are redrawn much earlier than before (where it could require
loading a different level set with different graphics to "repair" the
damaged global border).