Sync fonts to the assets dir so relative links in combined CSS will work

app.yml

SilverStripe\Assets\File:
  allowed_extensions:
    - eot
    - svg
    - ttf
    - woff
    - woff2

PageController.php

<?php

use SilverStripe\CMS\Controllers\ContentController;
use SilverStripe\Core\Flushable;
use SilverStripe\Core\Path;
use SilverStripe\View\Requirements;
use SilverStripe\View\ThemeResourceLoader;

class PageController extends ContentController implements Flushable
{
    public static function flush()
    {
        $loader = ThemeResourceLoader::inst();
        $paths = $loader->getThemePaths();
        $paths = array_reverse($paths);

        // Finds all files in webfonts directory, not just fonts.
        $fonts = [];
        foreach ($paths as $path) {
            if (substr($path, 0, 6) === 'vendor') {
                continue;
            }
            $fontDir = Path::join(BASE_PATH, $path, 'webfonts');

            if (is_dir($fontDir)) {
                $files = scandir($fontDir);
                if ($files !== false) {
                    foreach ($files as $file) {
                        if ($file === '.' || $file === '..') {
                            continue;
                        }
                        $filePath = Path::join($fontDir, $file);

                        // Only find files (and therefore ignore . and .. directories)
                        if (is_file($filePath)) {
                            $fonts[] = [$file, $filePath];
                        }
                    }
                }
            }
        }

        $webfontsOutPath = Path::join(ASSETS_PATH, 'webfonts');

        if (!file_exists($webfontsOutPath)) {
            mkdir($webfontsOutPath, 0777, true);
        }

        if (!is_dir($webfontsOutPath)) {
            user_error("Couldn't create webfonts folder in combinedFiles, it exists as a file already");

            return;
        }

        foreach ($fonts as $font) {
            [$name, $path] = $font;

            $outPath = Path::join($webfontsOutPath, $name);

            $shouldCopy = true;
            if (file_exists($outPath)) {
                if (filemtime($outPath) > filemtime($path)) {
                    $shouldCopy = false;
                } else {
                    unlink($outPath);
                }
            }

            if ($shouldCopy) {
                copy($path, $outPath);
            }
        }

        $existingFiles = scandir($webfontsOutPath);

        $fontNames = array_column($fonts, 0);
        foreach ($existingFiles as $existingFile) {
            if ($existingFile === '.' || $existingFile === '..') {
                continue;
            }

            $existingFilePath = Path::join($webfontsOutPath, $existingFile);

            if (is_file($existingFilePath)) {
                if (!in_array($existingFile, $fontNames)) {
                    unlink($existingFilePath);
                }
            }
        }
    }
}

That note about it taking 7ms, thats testing on our not-so-fast dev server in php7.1