Slightly expand pixel history column widths

* Should make exponential float values more obvious/visible.
Handle B8G8R8A8 as R8G8B8A8 in pixel history. Refs #110

* We can't create a UAV of format B8G8R8A8, so just make it R8G8B8A8 and
  let the GPU do the swizzle (everything else is exactly the same).