Grid placement for v3.0.x

[talaj]

Port of #3847 to v3.0.x, including fixes from #3855.

cc @kocio-pl

[codecov]

Codecov Report

Merging #3858 into v3.0.x will increase coverage by 0.08%.
The diff coverage is 93.12%.

@@            Coverage Diff             @@
##           v3.0.x    #3858      +/-   ##
==========================================
+ Coverage   65.28%   65.37%   +0.08%     
==========================================
  Files         443      444       +1     
  Lines       21815    21932     +117     
==========================================
+ Hits        14243    14337      +94     
- Misses       7572     7595      +23

Impacted Files	Coverage Δ
include/mapnik/text/symbolizer_helpers.hpp	0% <ø> (ø)	⬆️
src/symbolizer_enumerations.cpp	100% <ø> (ø)	⬆️
include/mapnik/text/text_properties.hpp	0% <ø> (ø)	⬆️
src/load_map.cpp	78.3% <100%> (+0.05%)	⬆️
src/text/text_properties.cpp	67% <60%> (-0.37%)	⬇️
src/text/symbolizer_helpers.cpp	95.57% <95.34%> (-0.37%)	⬇️
include/mapnik/grid_vertex_converter.hpp	96.05% <96.05%> (ø)
...clude/mapnik/geometry/polygon_vertex_processor.hpp	0% <0%> (-100%)	⬇️
src/image_util.cpp	39.48% <0%> (+0.46%)	⬆️

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[kocio-pl]

@artemp This change (together with #3846 bugfix) is quite interesting for me for testing and deploying on OSMF servers, since v3.0.18 is flawed. Do you plan to release v3.0.19 in some near future?

[artemp]

@kocio-pl - the short answer is yes.