preferred item image size for theme, get smallest photo near scale reque...

...sted

themes can declare preferred image size for images in items, with

    $a->theme_info = array(
        "item_image_size" => 1
    );

0: original, 1:640px, 2:320px

this works only for photos in friendica

mod/photo.php returns the nearest scaled image near requested one
if it doesn't exists, instead of nothing