Restore backspace

Now we're feature complete with the previous version in C, and even a
bit further (resize support).

zig/sdl/hello_sdl.zig

                        c.SDLK_ESCAPE => {
                            quit = true;
                        },
                        c.SDLK_BACKSPACE => {
                            pos = if (pos == 0) max_chars - 1 else (pos - 1) % (max_chars - 1);
                            msg[pos] = '_';
                        },
                        else => {},
                    }
                },