Add more filter options for image scaling

This PR adds some more filter constants to the extension. (http://www.php.net/manual/en/imagick.constants.php#imagick.constants.filters)

New filters you're able to use:
`imagick::FILTER_UNDEFINED` (to use the default filter)
`imagick::FILTER_HANNING`
`imagick::FILTER_HAMMING`
`imagick::FILTER_BLACKMAN`
`imagick::FILTER_LANCZOS` (especially for downsizing)
`imagick::FILTER_BESSEL`
`imagick::FILTER_SINC`

I also changed the default type of filter to `Undefined`