@kilbith spotted correctly that I had accidentally removed the
"soft" edging on the minimap overlay by converting it from RGBA
to Indexed, which killed the transparent pixels on the edging.
Related: #3730
This adds a simple, and small "North" indicator to the circular
minimap. The indicator is in a classical triangle-like arrow with a
little bit of shading to accentuate the shape and give it a little
bit depth. The indicator is stuck exactly at the edge as far outwards
as possible, and is not too intrusive but still easy enough to spot.
