Treat langtags in SVG switch case-insensitively

Treat langtags in SVG switch case-insensitively

See https://tools.ietf.org/html/bcp47#section-2.1.1

Also implement matching of systemLanguage attribs as
specified in the SVG spec

Note that librsvg that we use for rendering pngs of svg
files has a bug, and matches languages in the following
way instead of what is implemented in SVG::getMatchedLanguage()

public function getMatchedLanguage( $userPreferredLanguage, array $svgLanguages ) {
  foreach ( $svgLanguages as $svgLang ) {
    if ($svgLang == $userPreferredLanguage) {
      return $svgLang;
    }
    $dashPosition = strpos( $userPreferredLanguage, '-' );
    if ( $dashPosition !== false ) {
      if (
        strtolower( substr( $svgLang, 0, $dashPosition ) ) ==
        strtolower( substr( $userPreferredLanguage, 0, $dashPosition ) )
      ) {
        return $svgLang;
      }
    }
    return null;
}

Bug: T154132
Change-Id: Ibff66a0844f0cecfae0260c6a7d20aeedc2849a2