Add WebP compatibility to GD

This PR implements the functions [`imagewebp`](http://de1.php.net/manual/en/function.imagewebp.php) and [`imagecreatefromwebp`](http://de1.php.net/manual/en/function.imagecreatefromwebp.php).

Closes #3141.

PS: This implementation has a third parameter for `imagewebp` (the quality in percent). It seems like PHP has the same parameter although it is not documented.