Make #includes match C++ Core Guidelines

[javagl]

There currently are some subtle inconsistencies regarding include directives. Specifically, the pattern of brackets/quotes for <system_includes> and "own_includes" is not applied consistently. The relevant comment that summarizes this is at #56 (comment)

This should be fixed, preferably in one, dedicated cleanup step.

[javagl]

When this is addressed, the include strategy in cesium-unreal should also be reviewed. The Unreal headers are heavily confused by any windows header that may be included before them (meaning that having any include that (transitively) includes a windows header may cause hundreds of compile errors). One strategy for alleviating this might be to 1. first include all Unreal headers, 2. then include system includes, 3, then include "internal" includes. C.f. https://google.github.io/styleguide/cppguide.html#Names_and_Order_of_Includes

[kring]

Yeah Unreal gets really grumpy when we include Windows header. I don't think changing the order of includes will solve it, because source code using Unreal types has issues too, it's not just the Unreal headers that are a problem.

In general we shouldn't have any reason to include windows.h in any header. Unreal has portable versions of most of what we'd want to use from windows.h anyway. If we need something unusual, including windows.h in a .cpp that doesn't depend on Unreal is fine. The reason we ran into problems with spdlog is because it has weird defaults that make it so that #include <spdlog/spdlog.h> without definining SPDLOG_COMPILED_LIB (or probably others if we're going for header-only) causes the entire implementation to be dumped into the header, and the implementation requires windows.h.

I'm mostly writing this down for future reference, because it was a pain to figure out.

[kring]

I've changed the title to reflect the fact that the action here is to move cesium-native to the include strategy recommended in the C++ Core Guidelines: https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#sf12-prefer-the-quoted-form-of-include-for-files-relative-to-the-including-file-and-the-angle-bracket-form-everywhere-else

More detail here: #56 (comment)
And here: #56 (comment)

[kring]

We've now done this, more or less. We should write issues for any remaining issues as we find them (or just fix them), but we can close this issue.