Oops! A trivial typo in obfuscate_bitmap() made the obfuscation

Oops! A trivial typo in obfuscate_bitmap() made the obfuscation
function rather less uniform-looking than I'd intended. I _thought_
it looked a bit fishy, but had assumed it was just the human
tendency to see patterns where none exist. Now fixed, and some real
test vectors confirm that this time the obfuscation function is
actually what I intended it to be.

This means that all masked game IDs generated before this revision
are now invalid. That's a shame, but the game is only a day old and
I think I can reasonably justify it as teething trouble.

[originally from svn r5883]