Fix horizontal scrolling text with wide characters in TexView widget

[gnojus]

Fixes scrolling lines with double with characters. Example, where the problem occurs without this fix:

package main

import (
	"github.com/rivo/tview"
)

func main() {
	app := tview.NewApplication()
	t := tview.NewTextView().SetWrap(false)

	t.SetText("line 1 😀😀 q w e r t y a b c\nline 2 ---- q w e r t y a b c")

	if err := app.SetRoot(t, false).SetFocus(t).Run(); err != nil {
		panic(err)
	}
}

Maybe a special character should be added instead of invisible half of double with character? Because now they are disappearing at the edges. Like this ⋯ (though vim and nano use >).

[rivo]

See #467 (comment)