Add Unifont-based text renderer

[mike42]

This change adds a text rendering plugin called UnifontPrintBuffer. I've placed it in an "Experimental" package to indicate that the API is not stable.

The example is:

<?php

require __DIR__ . '/../vendor/autoload.php';

use Mike42\Escpos\PrintConnectors\FilePrintConnector;
use Mike42\Escpos\Printer;
use Mike42\Escpos\Experimental\Unifont\UnifontPrintBuffer;

$connector = new FilePrintConnector("php://stdout");
$printer = new Printer($connector);

// Use Unifont to render text
$unifontBuffer = new UnifontPrintBuffer("/usr/share/unifont/unifont.hex");
$printer -> setPrintBuffer($unifontBuffer);

// Most simple example
$printer->text("Hello\n");
$printer->setUpsideDown(true);
$printer->text("World\n");
$printer->cut();
$printer->close();

Which renders like this - demonstrating that text formatting commands work:

Key points are-

• This method is only suitable for printers which support user-defined fonts at 9x17 size - tested on Epson TM-series thermal printers.
• This is also only suitable for scripts where code points map easily to glyphs - eg. contextual forms, composing characters and bi-directional text will not render correctly.
• The user must supply the path to a GNU Unifont hex file, which is freely available.

Ref:

• Blog post: Using custom fonts to add new glyphs to an Epson printer
• Include UnifontPrintBuffer #813