Small cleanup for indexed images
Small cleanup for indexed images

This improves type stability but probably has no functional consequence

cdsousa/Images.jl,JuliaPackageMirrors/Images.jl,c42f/Images.jl,c42f/Images.jl,benmoran/Images.jl,cdsousa/Images.jl,benmoran/Images.jl,timholy/Images.jl