img: Correct bug in image size calculation code.

img: Correct bug in image size calculation code.

If an image was resized smaller, with width and height specified to values
that did not fit its aspect ratio, the image tag with/height were not
adjusted to the actual size imagemagick chooses.

This was broken by 03449610d6c666ba24bea68f01d896613e522278.

To fix it right, it unfortunatly needs to always read the src image now,
in order to determine if the image is being displayed larger, or resized
smaller. When resized smaller, it then always uses the size of the
thumbnail, while for larger it calculates the size.

(Only way to get rid of this sometimes extra image read would be to change
it to not allow displaying images larger.)