RFC: wined3d: Check for and use FBO blits early in BltOverride

[Chris Robinson]

IDirect3DDevice9::StretchRect is used to stretch-blit between video memory 
surfaces. It's implemented by calling IWineD3DSurfaceImpl_Blt, which itself 
will attempt IWineD3DSurfaceImpl_BltOverride to accelerate it. However, 
BltOverride will not accelerate the blit if neither surface is a swapchain or 
active render target 0, and will fall back to a sysmem->sysmem blit with 
possible conversions. This seems to happen in between every post-processing 
(screen-space shader) pass with Oblivion Graphics Extender and Morrowind 
Graphics Extender, which causes a severe loss in performance.

After some testing and talking with Henri, it seems a number of checks aren't 
needed if FBO blits are available, particularly the swapchain/active render 
target checks. The attached patch makes BltOverride check for FBO blits 
earlier, which helps it catch more cases where blits can be accelerated. It 
provides a significant improvement with the aforementioned programs.

I'm not sure of the full consequences of this move, however, particularly with 
earlier DX versions. Hence a request for comments.
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