perf(vt100) increase render performance for `tui_term` codepath

[chris-olszewski]

Description

The crate tui_term renders the vt100::Screen by fetching each cell and then filling the ratatui::buffer:Cell with the vt100 cell's contents.

This leads to the following functions being called once for each cell on the users terminal:

• Cell::contents which unnecessarily allocates on every call even though there's a fixed limit to the size of the str that could be returned. This PR changes this method to return a &str that is constructed from the content bytes. The new helper function append_char is basically String::push
• Grid::visible_row which iterates through the scrollback rows and screen rows. This PR changes this to instead index directly into the rows

Testing Instructions

Tested by profiling this basic script that render a tui_term 1000 times.

use std::io;

use ratatui::{
    prelude::CrosstermBackend,
    style::{Style, Stylize},
    widgets::{Block, Borders, Widget},
    Terminal,
};
use tui_term::widget::PseudoTerminal;

pub fn main() {
    let input = include_bytes!("turbo-build.log");
    let mut terminal =
        Terminal::new(CrosstermBackend::new(io::stdout())).unwrap();
    terminal.hide_cursor().unwrap();
    let size = terminal.size().unwrap();

    let mut parser =
        turborepo_vt100::Parser::new(size.height, size.width, 1024);
    parser.process(input);

    let screen = parser.screen();

    for _ in 0..1000 {
        let block = Block::default()
            .borders(Borders::ALL)
            .border_style(Style::default().blue());
        let term = PseudoTerminal::new(screen).block(block);
        terminal
            .draw(|frame| {
                let area = frame.size();
                term.render(area, frame.buffer_mut())
            })
            .unwrap();
    }
}

The two functions that this targets:

fill_buf_cell codepath Before

After

visible_cell codepath Before

After

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