fix bug 21400: unit::absolute_image being used for unit animations

When an animation frame had no defined image, it was using "absolute_image"
which yields the unit's `image_icon` if available, and `image` otherwise.

This causes undesirable behavior in cases highlighted by doofus1.

We fix it by replacing use of absolute image with an expression given by Coffee,
which prioritizes `image` over `image_icon`.