[Screen/Core] implement RenderTarget/RenderWindow/RenderTexture

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for now, only a basic RenderWindow class exists for displaying a simple window. 
This class only contains methods :

init
isRunning
swapBuffers

The goal of this enhancement is to provide better and standard services to 
manipulate render targets (not only render windows but also render textures and 
multi render target (MRT))

For now, MRT will not be implemented

global architecture :

RenderWindow -------|
                    |
RenderTexture ------|-----|> RenderTarget ------|> Resource<>
                    |
MultiRenderTarget --|

non-exhaustive RenderTarget method list :

//size
getWidth
getHeight

//pixel format
getPixelFormat
setPixelFormat

//camera handling
addCamera
getCamera
useCamera
removeCamera
removeAllCamera
...

//buffer swaps
waitForVSync
swapBuffers

//FPS counter
...

non-exhaustive RenderWindow method list :

//full-screen
setFullScreen
isFullScreen

//position and size
resize
reposition

//state
isVisible
setVisible
isClosed
...

Original issue reported on code.google.com by thoratou@gmail.com on 26 Jul 2010 at 11:43