SrcSet group's attribute scope

Hi Spinaker,

I don’t have any experience with using MetaSlider, but it looks like they surface their image tag attributes with the metaslider_{type}_image_attributes filter. You can generate the srcset value attribute by using TNSrcSet::get_image_srcset( $attachment_id, $size );. Example usage may be something like the following:

add_filter( 'metaslider_flex_slider_image_attributes', 'spinaker_metaslider_srcset_attribute', 10, 3 );
function spinaker_metaslider_srcset_attribute( $attributes, $slide, $slider_id ) {
    // $attachment_id and $size will need to be replaced by your own data
    $attributes['srcset'] = TNSrcSet::get_image_srcset( $attachment_id, $size );
    return $attributes;
}