paint something and then erase it with cairo

[Jie Bao <tzlfjz yahoo com cn>]

My application need to draw a line on a widget when mouse button is down, and then erase it and restore when 
mouse button is up. Previously I use GC to do all the work, I just call m_gc->set_function( Gdk::XOR), paint 
the line when mouse button is down, and paint it again when mouse button is up. Currently I try to port every 
thing to cairo, seems the cairo xor is not a bitwise XOR operator, so I need to do the work in some other 
way. So, with cairo, how can I paint something on a background, and then erase it and restore the background ?

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