Code assumed all images used for the crosshair have a size of 21x21.
The default image though is a PNG of a pre-rendered crosshair with a shadow,
which thus is not of that size.
That pre-rendered image, crosshairs-darkened.png, also had a size of 32x32,
while the crosshair in it had a pixel width of 1, so it's center would
not be pixel aligned with the cross lines. Image now resized to have
crosshair in the middle pixels.

Crosshair now is centered on middle, tested e.g. with 0, 0 lonlat.