This PR explicitly declares integer type for `image_center_offs`, which
formalizes accidentally implemented correct behavior. Previously type
was float, all rhs types in expression were integers, so result was int
cast to float.
Comment is also added clarifying why integer must be used - float may
cause images to be interpolated, even when translation is set to 0.
There should be no functional changes.
Pull Request: https://projects.blender.org/blender/blender/pulls/133573
bf19960937