Fix D3D Real XFB texture sampling.

Under the current D3D Real XFB system (which I don't really understand), when the texture lookup takes place, the x-coordinates passed in (x.5) are multiplied by 2, resulting in looking up each pixel about half a pixel too far to the right. This causes graphical errors on NVIDIA hardware due to every fifth column rounding up and choosing the wrong pixel. This fork compensates for incorrect texture coordinates by subtracting one half afterwards.