afford larger full_index, to save a few milliseconds during rendering (notably text_overview);

--- a/src/Pure/PIDE/document.scala	Thu Apr 03 22:04:57 2014 +0200
+++ b/src/Pure/PIDE/document.scala	Fri Apr 04 10:41:53 2014 +0200
@@ -187,7 +187,7 @@
         }
       }
 
-      private val block_size = 1024
+      private val block_size = 256
     }
 
     final class Commands private(val commands: Linear_Set[Command])