Greetings that use callback function runs multiple times

[jcubic]

Issue summary

When you use greetings as a function can call callback function. The greetings is rendered multiple times. 2 times on init and on every resize. Found by @leif in #853

Expected behavior

It should render once

Actual behavior

it renders multiple times

Steps to reproduce

Use this code:

const term = $('body').terminal({}, {
    greetings(cb) {
        cb(`This Terminal has ${this.cols()} cols`);
    }
});

Browser and OS

All

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