fixed #507 (limit persistent block zoom to 12x)

thanks, @cycomachead !

gui.js

@@ -3493,7 +3493,7 @@ IDE_Morph.prototype.userSetBlocksScale = function () {
     new DialogBoxMorph(
         null,
         function (num) {
-            myself.setBlocksScale(num);
+            myself.setBlocksScale(Math.min(num, 12));
         }
     ).withKey('zoomBlocks').prompt(
         'Zoom blocks',

history.txt

@@ -2195,3 +2195,4 @@ ______
 * Blocks: change the color of the %pause symbol to be more yellowish
 * Threads: fixed #506, thanks @haritop, for both the report and for providing the fix!!
 * GUI: fixed #412 (incomplete sprite-removal)
+* GUI: fixed #507 (limit persistent block zoom to 12x), thanks Michael!