Render has off-by-one error when using previous buffer with `diffLines`

[andyburris]

Can't exactly pin down the fix, but here's a test program that demonstrates it:

import { useState } from 'react'
import ReactCurse, { Text, useInput } from 'react-curse'

const App = () => {
  const [counter, setCounter] = useState(0)

  useInput(
    input => {
      if (input === 'q') ReactCurse.exit()
      else setCounter(counter + 1)
    },
    [counter]
  )
  return <Text>
    <Text block>{counter % 3 === 0 ? "x   " : ""}test 1</Text>
    <Text block>{counter % 3 === 1 ? "x   " : ""}test 2</Text>
    <Text block>{counter % 3 === 2 ? "x   " : ""}test 3</Text>
  </Text>
}

ReactCurse.render(<App />)

and I found that it can be worked around by changing

react-curse/term.ts

Lines 197 to 205 in f4dc11e

	const diffLine = full
	? line
	: line
	.map((i: Char, x: number) => {
	const [prevChar, prevModifier] = prevLine && prevLine[x] ? prevLine[x] : [' ', {}]
	const [char, modifier] = i
	return prevChar !== char || JSON.stringify(prevModifier) !== JSON.stringify(modifier) ? i : null
	})
	.filter(i => i !== undefined)

to just:

    const diffLine = line

so it's something about how diffLine is being used when it's null