Allow use of em in image spec sizes

Allow use of em in image spec sizes

* src/image.c (image_get_dimension): New function.
(compute_image_size): Use image_get_dimension to set the sizes, and
pass in the image struct instead of just the spec.
(image_set_transform):
(imagemagick_load_image):
(svg_load_image): Use the image instead of the spec in compute_image_size.
(syms_of_image): Add 'em' as a symbol.