fix PIXOFF based on cff29e546a

lib/rpl/types/grob.rb

@@ -11,7 +11,7 @@ class BitArray
 
   def []=(position, value)
     if value.zero?
-      @mask ^= (1 << position)
+      @mask &= ~(1 << position)
     else
       @mask |= (1 << position)
     end

lib/rpl/words/graphics.rb

@@ -117,7 +117,7 @@ module RplLang
                                  x = args[1].value.to_i
                                  y = args[0].value.to_i
 
-                                 @lcd_grob.set_pixel(x, y, 0) # FIXME: toggle pixel instead of turning it off
+                                 @lcd_grob.set_pixel(x, y, 0)
                                end )
         @dictionary.add_word!( ['pix?'],
                                category,