DDRAW: Fix color keying in DIB_DirectDrawSurface_Blt

[H. Verbeet]

The default value for keylow in DIB_DirectDrawSurface_Blt was 0, but
should be 0xFFFFFFFF. This fixes the in-game intro movie for
BattleZone II. Also optimize DIB_DirectDrawSurface_Blt so it takes the
"easy" path when DDBLT_DDFX is set but lpbltfx is NULL or contains an
empty dwDDFX;

Changelog:
 - Fix the default value for keylow in DIB_DirectDrawSurface_Blt
 - Unset DDBLT_DDFX when lpbltfx is NULL / contains no dwDDFX;
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