Drop special handling of rotation from MapCanvasMap::paint

Stop assuming absence of rotation from MapCanvasItem::setRect,
but rather assume that the passed rectangle purely encodes
origin and size by only looking at upperLeft corner (xmin,ymax)
and width/height of it, scaled by current map resolution.

Makes the code simpler (maybe) and still fixes the regression.
Also fixes glitches on zoom while map is rotated.

Makes glitches on rotation worst, but again that's not a real
regression.