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

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

mod/photo.php returns the nearest scaled image near requested one
if it doesn't exists, instead of nothing
Merge pull request #1062 from fabrixxm/item_image_size_per_theme

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