GUI: Limit Zoom blocks scale to max of 12; fixes #255

Simple and easy fix. :)

Even if you type '129999999999' in the display box, it won't draw blocks larger than 12x scale. 

(Of course, someone who really wanted could always modify the localStorage setting, but then s/he is asking for trouble!)
I somewhat disagree because, if I try '12222222200000000000' it actually seems to limit the scale somewhere around 50. Thus you get a different preview than what actually happens.