strel_type: ignore OffsetArray wrapper

[johnnychen94]

julia> using OffsetArrays: centered, no_offset_view

julia> se = centered(no_offset_view(strel_diamond((3, 3))))
3×3 OffsetArray(::ImageMorphology.StructuringElements.SEDiamondArray{2, 2, UnitRange{Int64}, 0}, -1:1, -1:1) with eltype Bool with indices -1:1×-1:1:
 0  1  0
 1  1  1
 0  1  0

julia> strel_type(se)
ImageMorphology.StructuringElements.SEMask{2}()

julia> strel_type(strel_diamond((3, 3)))
ImageMorphology.StructuringElements.SEDiamond{2, 2, UnitRange{Int64}}((-1:1, -1:1), (1, 2), 1