Compression issues

[adrg]

Fixes various scenarios in which the optimizer freezes or distorts the original PDF images.

Resolves #322

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This change is 

[gunnsth]

Reviewed 5 of 5 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved

[codecov]

Codecov Report

Merging #331 into v3 will decrease coverage by 0.14%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##               v3     #331      +/-   ##
==========================================
- Coverage   38.68%   38.54%   -0.15%     
==========================================
  Files         122      122              
  Lines       23083    23167      +84     
==========================================
  Hits         8930     8930              
- Misses      12872    12956      +84     
  Partials     1281     1281

Impacted Files	Coverage Δ
pdf/model/xobject.go	0% <0%> (ø)	⬆️
pdf/model/image.go	12.92% <0%> (-1.05%)	⬇️
pdf/model/optimize/image_ppi.go	0% <0%> (ø)	⬆️
pdf/core/encoding.go	36.43% <0%> (-2.14%)	⬇️
pdf/model/colorspace.go	6.3% <0%> (ø)	⬆️

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