add missing Blitbuffer since it's removed from global space

frontend/document/credocument.lua

@@ -1,4 +1,5 @@
 local Geom = require("ui/geometry")
+local Blitbuffer = require("ffi/blitbuffer")
 local CreOptions = require("ui/data/creoptions")
 local Document = require("document/document")
 local Geom = require("ui/geometry")

frontend/document/document.lua

@@ -1,4 +1,5 @@
 local DrawContext = require("ffi/drawcontext")
+local Blitbuffer = require("ffi/blitbuffer")
 local Cache = require("cache")
 local CacheItem = require("cacheitem")
 local TileCacheItem = require("document/tilecacheitem")

frontend/ui/widget/textboxwidget.lua

@@ -1,3 +1,4 @@
+local Blitbuffer = require("ffi/blitbuffer")
 local Widget = require("ui/widget/widget")
 local RenderText = require("ui/rendertext")
 local Screen = require("ui/screen")
@@ -71,7 +72,7 @@ However string.gmatch() has one significant disadvantage for me:
 You can't split a string while matching both the delimited
 strings and the delimiters themselves without tracking positions
 and substrings. The string.gsplit() function below takes care of
-this problem. 
+this problem.
 Author: Peter Odding
 License: MIT/X11
 Source: http://snippets.luacode.org/snippets/String_splitting_130