Fix -Wsubobject-linkage warning

[buzzhuzz]

Description

[8/476 0.2/sec] Building CXX object src/CMakeFiles/OrcaSlicer_profile_validator.dir/OrcaSlicer_profile_validator.cpp.o
In file included from /opt/packages/3d/OrcaSlicer/src/libslic3r/GCode.hpp:26,
                 from /opt/packages/3d/OrcaSlicer/src/libslic3r/calib.hpp:5,
                 from /opt/packages/3d/OrcaSlicer/src/libslic3r/AppConfig.hpp:12,
                 from /opt/packages/3d/OrcaSlicer/src/libslic3r/PresetBundle.hpp:5,
                 from /opt/packages/3d/OrcaSlicer/src/OrcaSlicer_profile_validator.cpp:3:
/opt/packages/3d/OrcaSlicer/src/libslic3r/GCode/SmallAreaInfillFlowCompensator.hpp:12:7: warning: ‘Slic3r::SmallAreaInfillFlowCompensator’ has a field ‘std::unique_ptr<{anonymous}::tk::spline> Slic3r::SmallAreaInfillFlowCompensator::flowModel’ whose type has internal linkage [-Wsubobject-linkage]
   12 | class SmallAreaInfillFlowCompensator
      |       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Having tk::spline header-only implementation included from SmallAreaInfillFlowCompensator.hpp makes
SmallAreaInfillFlowCompensator::flowModel have separate (albeit the same) implementation in each translation unit.

In order to fix this issue, SmallAreaInfillFlowCompensator::flowModel converted to opaque 'pimpl'

Tests

Fixes ~240 compilation warnings

[buzzhuzz]

@mjonuschat , could you please help review this change?

[buzzhuzz]

Just found that PR #5963 adds warning suppression using pragmas. Unfortunately, this leads to -Wunknown-warning warnigs on clang builds:

/opt/3d/OrcaSlicer/src/libslic3r/GCode/SmallAreaInfillFlowCompensator.hpp:17: warning: unknown warning group '-Wsubobject-linkage', ignored [-Wunknown-warning-option]
In file included from /opt/3d/OrcaSlicer/src/libslic3r/GCode/SmallAreaInfillFlowCompensator.cpp:17:
/opt/3d/OrcaSlicer/src/libslic3r/GCode/SmallAreaInfillFlowCompensator.hpp:17:32: warning: unknown warning group '-Wsubobject-linkage', ignored [-Wunknown-warning-option]
   17 | #pragma GCC diagnostic ignored "-Wsubobject-linkage"
      |                                ^

Which adds extra points to this PR

[SoftFever]

Thanks, it looks good.

I'm not very clear about the purpose of TK_SPLINE_OUTER_NAMESPACE macro, could you explain it?

[buzzhuzz]

Types within anonymous namespace cannot be forward declared.

I've added a macro to keep an option to include spline.h within anonymous workspace if needed.

[SoftFever]

Would it be simpler if we move the spline out of the anonymous namespace?
It's still in tk namespace, so I think it won't cause any issue

[buzzhuzz]

That would also work.

[SoftFever]

Looks good
Thank you