Fix double-rendering of partially-selected tokens, causing visual bolding

[bobbylight]

This prior PR added ligature support to RSTA, but had a visual issue that was harder to notice on macOS, but is pretty apparent on Windows. Specifically, when kerning or ligatures were enabled, partially-selected tokens could appear slightly bolded due to them effectively being double-rendered. This results in pixels with < 1 alpha to appear darker than they should.

This PR addresses that issue by never rerendering chars on top of themselves after they are rendered once. This does require our rendering logic for selected text to be more complicated (actually, somewhat similar to what it was before ligature support); however I've also taken the opportunity to share more code between SyntaxView and WrappedSyntaxView here, so the pros outweigh the cons :)

Before	After

[codecov]

Codecov Report

Attention: Patch coverage is 83.01887% with 9 lines in your changes missing coverage. Please review.

Project coverage is 74.62%. Comparing base (7055442) to head (09857b2).

Files	Patch %	Lines
...n/java/org/fife/ui/rsyntaxtextarea/SyntaxView.java	85.71%	4 Missing and 2 partials ⚠️
...va/org/fife/ui/rtextarea/RTextAreaHighlighter.java	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #559      +/-   ##
============================================
+ Coverage     74.61%   74.62%   +0.01%     
  Complexity     6881     6881              
============================================
  Files           178      178              
  Lines         30267    30264       -3     
  Branches       3919     3912       -7     
============================================
+ Hits          22584    22586       +2     
+ Misses         5860     5857       -3     
+ Partials       1823     1821       -2     

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