Commit my replacement Windows I-beam mouse pointer.

Installing this systemwide as the Windows text selection cursor is a
workaround for 'black-pointer'. It's a white I-beam with a one-pixel
black outline around it, so it should be visible on any background
colour. (I suppose that a backdrop of tightly packed I-beams looking
just like it might successfully hide it, but that's unlikely :-)

I constructed this some years ago for personal use; I needed it again
this week and had to go and recover it from a backup of a defunct
system, which made me think I really ought to check it in somewhere,
and this 'contrib' directory seems like the ideal place.