Scrolling performance - redraw old quotes.

Rather than fetching quotes each time we need to redraw them becuase the
user has scrolled the screen, just redraw the old quotes.

cmd/mop/main.go

@@ -168,7 +168,7 @@ loop:
 		}
 
 		if redrawQuotesFlag && len(keyboardQueue) == 0 {
-			screen.Draw(quotes)
+			screen.DrawOldQuotes(quotes)
 			redrawQuotesFlag = false
 		}
 	}

screen.go

@@ -119,6 +119,12 @@ func (screen *Screen) ScrollBottom(max int) {
 	}
 }
 
+func (screen *Screen) DrawOldQuotes(quotes *Quotes) {
+    screen.draw(screen.layout.Quotes(quotes), true)
+    termbox.Flush()
+}
+
+
 // Draw accepts variable number of arguments and knows how to display the
 // market data, stock quotes, current time, and an arbitrary string.
 func (screen *Screen) Draw(objects ...interface{}) *Screen {