Fix Painter:viewport()

This works because Painter inherits from ViewRect. Unsure how this went unnoticed.

docs/changelog.txt

@@ -34,6 +34,7 @@ changelog.txt uses a syntax similar to RST, with a few special sequences:
 # Future
 
 ## Lua
+- ``gui.Painter``: fixed error when calling ``viewport()`` method
 - `xlsxreader`: Added Lua class wrappers for the xlsxreader plugin API
 
 ## Documentation

library/lua/gui.lua

@@ -246,7 +246,7 @@ function Painter:isValidPos()
 end
 
 function Painter:viewport(x,y,w,h)
-    local vp = ViewRect.viewport(x,y,w,h)
+    local vp = ViewRect.viewport(self,x,y,w,h)
     vp.cur_pen = self.cur_pen
     vp.cur_key_pen = self.cur_key_pen
     return mkinstance(Painter, vp):seek(0,0)