Blender Git Loki

Cleanup: spelling (correct c5c8c68eec93e57ed46be4371d8831e2f0fe3fe2)

"iff" was intended as "if and only if". while exact use of abbreviations
isn't clear cut, I assumed this was a typo & it's not used anywhere
else in source/, expand to "only if" (suggested by Sybren).