offset generic array back to 1-based indexing array

[johnnychen94]

Some image processing operations will produce OffsetArray as an output, in which case convert(Array{T, N}, img::OffsetArray) does not work.

For example,

using ImageTransformations, TestImages, Sixel
img = testimage("camera")
imgr = imrotate(img, 0.4) # 672×672 OffsetArray(::Array{Gray{N0f8},2}, -79:592, -79:592)
sixel_encode(imgr) # errors

With this PR:

[codecov]

Codecov Report

Merging #12 (205b669) into master (dc3f05f) will increase coverage by 0.36%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #12      +/-   ##
==========================================
+ Coverage   38.05%   38.41%   +0.36%     
==========================================
  Files          10       10              
  Lines         339      341       +2     
==========================================
+ Hits          129      131       +2     
  Misses        210      210              

Impacted Files	Coverage Δ
src/Sixel.jl	100.00% <ø> (ø)
src/encoder.jl	87.50% <100.00%> (+1.78%)	⬆️

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