feat!: Implement a font registry for text rendering

[meyfa]

Fixes #82
Fixes #168
Fixes #171

This patch removes the artificial SVGFont class as well as the SVGText::setFont(SVGFont $font) method. Instead, it is now possible to register TrueType (.ttf) font files by calling SVG::addFont() with a file path.

A TrueType font file parser is implemented based on Microsoft's OpenType specification. This allows us to choose the best matching font file for a given text element automatically, just like a browser would, based on algorithms from CSS.

Currently, TTF files are supported with all weight levels, and in the italic and non-italic styles. The simple algorithm implemented here tries to match the font family exactly, if possible, falling back to any other font. Generic font families such as "serif" or "monospace" aren't supported yet and will trigger the fallback. Also, the relative weights "bolder" and "lighter" aren't computed and fall back to normal weight.

BREAKING CHANGE: SVGFont class and some methods on SVGText removed.

[meyfa]

For reference, here is a sample image:

Generated by this code:

<?php
require __DIR__ . '/vendor/autoload.php';
use SVG\SVG;

const FONTS_DIR = __DIR__.'/fonts/';

SVG::addFont(FONTS_DIR . 'Ubuntu-Regular.ttf');
SVG::addFont(FONTS_DIR . 'Ubuntu-Bold.ttf');
SVG::addFont(FONTS_DIR . 'Ubuntu-Italic.ttf');
SVG::addFont(FONTS_DIR . 'Ubuntu-BoldItalic.ttf');
SVG::addFont(FONTS_DIR . 'Ubuntu-Light.ttf');
SVG::addFont(FONTS_DIR . 'Ubuntu-LightItalic.ttf');
SVG::addFont(FONTS_DIR . 'UbuntuMono-Regular.ttf');
SVG::addFont(FONTS_DIR . 'UbuntuMono-Bold.ttf');
SVG::addFont(FONTS_DIR . 'UbuntuMono-Italic.ttf');
SVG::addFont(FONTS_DIR . 'UbuntuMono-BoldItalic.ttf');

$image = SVG::fromString('
<svg width="220" height="220">
  <rect x="0" y="0" width="100%" height="100%" fill="lightgray"/>
  <g font-size="15">
    <text y="20">regular</text>
    <text y="40" font-weight="100">light</text>
    <text y="60" font-weight="100" font-style="italic">light italic</text>
    <text y="80" font-weight="bold">bold</text>
    <text y="100" font-style="italic">italic</text>
    <text y="120" font-weight="bold" font-style="italic">bold italic</text>
    <text y="140" font-family="Ubuntu Mono">monospace</text>
    <text y="160" font-size="15" font-family="Ubuntu Mono" font-weight="bold">monospace bold</text>
    <text y="180" font-size="15" style="font-family: Ubuntu Mono; font-style: italic">monospace italic</text>
    <text y="200" font-size="15" style="font-family: Ubuntu Mono; font-weight: bold; font-style: italic">monospace bold italic</text>
  </g>
</svg>
');

header('Content-Type: image/png');
$doc = $image->getDocument();
imagepng($image->toRasterImage($doc->getWidth(), $doc->getHeight()));