glsl: always initialize image_* fields, copy them on interface init

glsl: always initialize image_* fields, copy them on interface init

Interfaces can have image properties set in case they are buffer
interfaces. Make sure not to lose this information.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>