> FEEDBACK: This is a bit weird with relative mode - the last two lines in relative mode should technically be -1, but I had to use -1.1 because -1 means use original value. The engine seems to bounds check the -1.1 to -1 anyway so it works, but it's not intuitive. 
Ashley maybe you could use another value instead of -1 to represent the original value? Or use < -1 instead of == -1?
You can't set the texture co-ordinates outside the range [0, 1], because that would show content outside the object's image. So any texture co-ordinates you set will be clamped to this range. -1 is not in that range, so is safe to use as a special value for "don't change".