Write out multi-encoder filters correctly

[peterwilliams97]

Fix for #279
Replaces #280 which was based on the wrong branch

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This change is 

[codecov]

Codecov Report

Merging #281 into development will increase coverage by 4.76%.
The diff coverage is 0.00%.

@@               Coverage Diff               @@
##           development     #281      +/-   ##
===============================================
+ Coverage        51.85%   56.62%   +4.76%     
===============================================
  Files              202      202              
  Lines            38509    38512       +3     
===============================================
+ Hits             19970    21807    +1837     
+ Misses           15990    13933    -2057     
- Partials          2549     2772     +223     

Impacted Files	Coverage Δ
core/encoding.go	50.65% <0.00%> (+12.03%)	⬆️
internal/jbig2/segments/text-region.go	46.85% <0.00%> (+0.26%)	⬆️
internal/jbig2/segments/symbol-dictionary.go	50.54% <0.00%> (+0.30%)	⬆️
model/writer.go	75.67% <0.00%> (+0.42%)	⬆️
internal/jbig2/segments/generic-region.go	47.66% <0.00%> (+1.35%)	⬆️
fdf/parser.go	40.27% <0.00%> (+1.38%)	⬆️
internal/jbig2/page.go	31.21% <0.00%> (+1.46%)	⬆️
model/reader.go	46.41% <0.00%> (+1.67%)	⬆️
contentstream/contentstream.go	76.36% <0.00%> (+1.81%)	⬆️
core/primitives.go	78.75% <0.00%> (+2.40%)	⬆️
... and 44 more

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[gunnsth]

LGTM