Fix: non-scrollable textview has strange behavior

[dzpao]

It seems that there are two lines of code that are upside down.

This should fix #341

[dzpao]

In order to explain the problem more clearly, I wrote a short example.

If you run the example with the master version, you can see that every time the screen is full, it will be emptied.

With patch applied, the screen will continue to scroll up without being emptied. I think this is more in line with people's expectations.

package main

import (
	"fmt"
	"time"

	"github.com/rivo/tview"
)

func main() {
	app := tview.NewApplication()
	textView := tview.NewTextView().
		SetScrollable(false).
		SetChangedFunc(func() {
			app.Draw()
		})

	go func() {
		i := 0
		for {
			fmt.Fprintf(textView, "%d\n", i)
			i++
			time.Sleep(200 * time.Millisecond)
		}
	}()

	textView.SetBorder(true)
	mainView := tview.NewFlex().
		SetDirection(tview.FlexRow).
		AddItem(textView, 10, 1, true)
	if err := app.SetRoot(mainView, true).Run(); err != nil {
		panic(err)
	}
}