- Restored "dither" option for conversion from float -> byte images.

This now is a post-process option only (used to be in render).
  It is only handled within the Imbuf/ module, on conversions from float
  to byte rect, which atm mostly happens on saving images.

- Small fix: when using Scene RenderLayer nodes, the speed vectors for
  these nodes were not created when that scene had "Do Composite" off.

source/blender/src/writeimage.c

@@ -136,6 +136,9 @@ static void save_rendered_image_cb_real(char *name, int zbuf)
 			ibuf->zbuf_float= rres.rectz;
 		}
 		
+		/* float factor for random dither, imbuf takes care of it */
+		ibuf->dither= G.scene->r.dither_intensity;
+		
 		BKE_write_ibuf(ibuf, str, G.scene->r.imtype, G.scene->r.subimtype, G.scene->r.quality);
 		IMB_freeImBuf(ibuf);	/* imbuf knows rects are not part of ibuf */