recorder: Always initialize prev when computing the run-length

We need to initialize prev when we handle the initial pixel in a
rectangle, or we may detect the following pixel as identical or different
when it's not.  This causes the top-left pixel in a rectangle to
occasionally be wrong leaving a trail of "dirty pixels" in the capture.

src/screenshooter.c

@@ -307,8 +307,8 @@ weston_recorder_frame_notify(struct wl_listener *listener, void *data)
 				} else {
 					p = output_run(p, prev, run);
 					run = 1;
-					prev = delta;
 				}
+				prev = delta;
 			}
 		}