Minor Clean Up of includes and gen_tile.cpp

[hummeltech]

Also:

• Use mapnik::freetype_engine::register_fonts in gen_tile.cpp rather than recursing manually
  • Available since Mapnik v2 which is no longer supported

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 93.87755% with 3 lines in your changes missing coverage. Please review.

Project coverage is 78.29%. Comparing base (5014494) to head (29314de).
Report is 59 commits behind head on master.

Files with missing lines	Patch %	Lines
src/gen_tile.cpp	93.47%	3 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #402      +/-   ##
==========================================
- Coverage   78.31%   78.29%   -0.03%     
==========================================
  Files          24       24              
  Lines        5866     5851      -15     
==========================================
- Hits         4594     4581      -13     
+ Misses       1272     1270       -2     

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