Fixed PdfColorspaceSpecialIndexed.ImageToRGB()

[peterwilliams97]

Fixes #258 which has a PDF file that shows the problem.

I fixed the indexing.

This fix will fail for indexes over colorspaces for which BitsPerComponent ≠8

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This change is 

[codecov]

Codecov Report

Merging #259 into development will increase coverage by 4.98%.
The diff coverage is 77.77%.

@@               Coverage Diff               @@
##           development     #259      +/-   ##
===============================================
+ Coverage        54.11%   59.09%   +4.98%     
===============================================
  Files              192      192              
  Lines            36884    36884              
===============================================
+ Hits             19958    21795    +1837     
+ Misses           14378    12318    -2060     
- Partials          2548     2771     +223

Impacted Files	Coverage Δ
model/colorspace.go	27.64% <77.77%> (+16.72%)	⬆️
internal/jbig2/segments/text-region.go	46.85% <0%> (+0.26%)	⬆️
internal/jbig2/segments/symbol-dictionary.go	50.54% <0%> (+0.3%)	⬆️
model/writer.go	75.67% <0%> (+0.42%)	⬆️
internal/jbig2/segments/generic-region.go	47.66% <0%> (+1.35%)	⬆️
fdf/parser.go	40.27% <0%> (+1.38%)	⬆️
internal/jbig2/page.go	31.21% <0%> (+1.46%)	⬆️
model/reader.go	46.41% <0%> (+1.67%)	⬆️
contentstream/contentstream.go	76.36% <0%> (+1.81%)	⬆️
core/primitives.go	78.75% <0%> (+2.4%)	⬆️
... and 44 more

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[gunnsth]

Minor comments

[gunnsth]

LGTM