Some X client optimizations.

- Only call cb_flush() if a new rect was queued.
- Ensure bitmap_drv_request_some_pixels() is only called once in
  response to an expose request.

This should fix issue #565.

src/conio/bitmap_con.c

@@ -722,11 +722,11 @@ static void blinker_thread(void *data)
 			screen->update_pixels = 0;
 			pthread_mutex_unlock(&screenlock);
 			cb_drawrect(rect);
+			cb_flush();
 		}
 		else {
 			pthread_mutex_unlock(&screenlock);
 		}
-		cb_flush();
 	}
 }
 

src/conio/x_events.c

@@ -805,10 +805,12 @@ static void expose_rect(int x, int y, int width, int height)
 		last = NULL;
 		bitmap_drv_request_some_pixels(sx, sy, ex-sx+1, ey-sy+1);
 	}
-	// Do nothing...
-	if (sx == ex || sy == ey)
-		return;
-	bitmap_drv_request_some_pixels(sx, sy, ex-sx+1, ey-sy+1);
+	else {
+		// Do nothing...
+		if (sx == ex || sy == ey)
+			return;
+		bitmap_drv_request_some_pixels(sx, sy, ex-sx+1, ey-sy+1);
+	}
 }
 
 static bool