VideoSoftware: Fix unsigned overflow bug in fog

Was causing fog errors on the left half of the screen.

Only appeared to affect visual studio builds, GCC did the correct
thing.

Source/Core/VideoBackends/Software/Tev.cpp

@@ -769,7 +769,8 @@ void Tev::Draw()
       // - scaling of the "k" coefficient isn't clear either.
 
       // First, calculate the offset from the viewport center (normalized to 0..1)
-      float offset = (Position[0] - (bpmem.fogRange.Base.Center - 342)) / (float)xfmem.viewport.wd;
+      float offset = (Position[0] - (static_cast<s32>(bpmem.fogRange.Base.Center) - 342)) /
+                     static_cast<float>(xfmem.viewport.wd);
 
       // Based on that, choose the index such that points which are far away from the z-axis use the
       // 10th "k" value and such that central points use the first value.