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!)
Let's not limit the preview, since it's not going to cause any irreversible changes to user's environment, I'm leaving out this line