Problems implementing double buffered painting for Windows

[Cecilio Pardo]

Hello,

I'm trying to implement double buffered painting for Windows, to
eliminate flicker. I am encountering some problems:

- When to flip buffers? I have tried to attach the flip to
  frame_up_to_date_hook on the 'terminal' struct, but it sometimes get
  called when it should not, producing flicker.

- What portion of the screen has been updated? I have seen no clear way
  to find this out. I'm resorting to intercept calls to w32_fill_rect to
  mark areas as invalidated, as it /seems/ to be always used to clean
  the area before painting, but this is clearly not a solution.

Any ideas welcome. Thank you.

-- 
Cecilio Pardo