Add bottomclip parameter to FillSimplePoly() for the software implementation

- Polygons will be clipped to bottomclip. If this is zero or below, they
  will be clipped to the bottom of the screen instead. This keeps the
  polygons from overwriting the status bar border for sofware 2D. The
  hardware version ignores it, since it always draws the status bar border
  every frame.

src/am_map.cpp

@@ -2046,7 +2046,8 @@ void AM_drawSubsectors()
 				scale / scaley,
 				rotation,
 				colormap,
-				floorlight
+				floorlight,
+				f_y + f_h
 				);
 		}
 	}