Added far clipping to visible sprite determination.
Added far clipping to visible sprite determination.

The flats (i.e., sprites) are now thrown out of the visible flats list
if both of the vertical edges' Z values are greater than the far
plane's. This would mean the flat is completely obscured by fog. The
visible sprite determination algorithm and the sprite rendering both
need a lot of polishing, but I'll figure it out eventually.

afritz1/OpenTESArena