Add text style rendering mode

[adrg]

Adds rendering mode field to the TextStyle structure. Also adds text chunks with different rendering modes to the StyledParagraph component.

Should resolve #221

[codecov]

Codecov Report

Merging #336 into v3 will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##               v3     #336      +/-   ##
==========================================
+ Coverage   57.73%   57.78%   +0.05%     
==========================================
  Files         145      145              
  Lines       25225    25214      -11     
==========================================
+ Hits        14563    14571       +8     
- Misses      10266    10276      +10     
+ Partials      396      367      -29

Impacted Files	Coverage Δ
pdf/creator/text_style.go	100% <ø> (ø)	⬆️
pdf/creator/const.go	100% <ø> (ø)	⬆️
pdf/creator/styled_paragraph.go	92% <100%> (+2.55%)	⬆️
pdf/model/colorspace.go	35.49% <0%> (-1.1%)	⬇️
pdf/contentstream/processor.go	59.14% <0%> (+0.3%)	⬆️
pdf/model/functions.go	49.11% <0%> (+1.1%)	⬆️
pdf/contentstream/creator.go	36.31% <0%> (+1.45%)	⬆️
pdf/contentstream/utils.go	28.84% <0%> (+9.61%)	⬆️

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