fix(core): report 0 loops for a still image - #126

$image->loops() on a still image throws with the vips driver:

$manager = new ImageManager(new Driver());
$manager->decodePath('photo.jpg')->loops(); // DriverException, vips_image_get: field "loop" not found
$manager->createImage(10, 10)->loops();     // same

GD returns 0 (its default), Imagick returns getImageIterations(), 0 for a still image. The vips Core::loops() read the loop field unguarded. It now checks the field first and returns 0 when it is missing, like count() does for n-pages.