Possible bug: zero-width-escapes at the end of a line are not rendered

Hi Jonathan,

I think I've come across a potential bug in prompt_toolkit.renderer._output_screen_diff: if a line of output is updated so it ends with a zero-width-escape fragment, the ZWE at the end of the line does not get rendered.

In the example app below, pressing u appends a text fragment and a ZWE fragment (red text) to the end of the output. However, the zwe fragment is not rendered since it is at the end of a line. If another text fragment is append (u), or the app is closed (q), the ZWE at the end of the line is rendered.

I'm using red text to represent a ZWE in this example, but my use case is using ZWEs to display terminal graphics.

from prompt_toolkit.application import Application
from prompt_toolkit.layout.layout import Layout
from prompt_toolkit.layout.containers import Window
from prompt_toolkit.layout.controls import FormattedTextControl
from prompt_toolkit.key_binding import KeyBindings

formatted_text = [
    ("", "start-of-line"),
]

kb = KeyBindings()

@kb.add("u")
def update(event):
    formatted_text.extend(
        [
            ("", " text-fragment"),
            ("[ZeroWidthEscape]", "\x1b[0;31m zwe-fragment\x1b[0m"),
        ]
    )

@kb.add("c-q")
def quit(event):
    event.app.exit()

app = Application(
    full_screen=False,
    layout=Layout(container=Window(FormattedTextControl(lambda: formatted_text))),
    key_bindings=kb,
)

app.run()

Run the provided prompt_toolkit example and inspect prompt_toolkit.renderer._output_screen_diff while pressing u to reproduce the missing zero-width escape at the line end. Confirm the fix by checking that the final zero-width-escape fragment is rendered immediately after the update, without needing another text fragment or application exit.